LAVENDER, Justice.
¶ 1 The dispositive issue is whether the district court has jurisdiction to grant declaratory relief to a taxpayer who, without first invoking any judicial remedy provided in the Ad Valorem Tax Code, commences an action, challenging the county assessor’s refusal to recognize an exemption claimed under a self-executing provision of the Oklahoma Constitution. We answer in the negative and hold that the legislatively prescribed judicial remedies afforded to these taxpayers under the Code are adequate and exclusive, and that the district court therefore lacked subject matter jurisdiction to entertain their declaratory judgment action.
¶ 2 Owners of an allegedly nonprofit housing project, providing subsidized and minimally-priced housing for elderly and handicapped people, were notified in 1994 by the County Assessor of Muskogee County, Jackie Scott, that their property would no [109]*109longer be listed on the tax roEs as exempt.1 They had acquired the property during the previous year and aEegedly continued to use it as the previous owner did for “charitable purposes” within the meaning of 68 O.S.1991 § 2887(9).2
¶3 In response to the assessor’s notice the owners lodged a complaint pursuant to 68 O.S.1991 § 2876(D),3 which gives taxpayers 20 days to object in writing to an action taken by the county assessor. The latter held a hearing and denied the owners’ protest. They then appealed to the county board of equalization, which also ruled against them.4 The terms of 68 O.S.1991 § 2880.1 give taxpayers and county assessors the right to appeal to the district court from an order of the county board of equalization ■within 10 days after final adjournment of the board.5 In eases where “the illegality of the tax is alleged to arise by reason of some action from, which the laws provide no appeal, the aggrieved person shall pay the full amount of the taxes at the time and in the manner provided by láw, and shall give notice to the officer collecting the taxes showing the grounds of complaint and that suit will be brought against the officer for recovery of them.” (Emphasis added.) 68 O.S. 1991 § 2886(A).
¶4 The owners complied with neither § 2880.1 nor § 2886. Instead, in October 1994, several months after the board’s decision, they sought declaratory relief in the district court, aHeging that the county assessor, “through [the] Muskogee County Board of Equalization,” refuses to recognize the exempt status of their property, despite its [110]*110nonprofit, charitable use. In May 1995 the trial court approved a partial compromise reached by the parties, whereby the owners agreed to pay the 1994 ad valorem tax without admitting to its validity, and the controversy would be converted into an appeal from a hypothetical adverse ruling by the board of equalization regarding the 1995 assessment.6
¶ 5 A hearing was eventually held in August 1995 to determine whether the property is exempt. Before evidence was introduced, however, counsel for the county assessor objected to the trial court’s exercise of jurisdiction and argued that the county assessor had never agreed to the terms of the compromise that had earlier been approved by the court. The trial judge rejected counsel’s assertions and allowed the owners to proceed.
¶6 At the hearing’s conclusion the trial court declared the property exempt, finding that it is “used exclusively for charitable purposes and that the facilities operated by Plaintiffs are open to any person regardless of ability to pay.” The court also found “no evidence that the owners received any remuneration.’.’ The county assessor appealed.
¶ 7 Only one issue was raised on appeal— whether the trial court had jurisdiction to resolve the exemption issue. The basis for asserting lack of jurisdiction was the owners’ failure to appeal to the district court under § 2880.1.7 In support of the exclusiveness of this statutorily prescribed judicial remedy the county assessor relied solely on the terms of 68 O.S.1991 § 2885(A):
A. The proceedings before the county assessor, boards of equalization and appeals therefrom shall he the sole method by which assessments or equalizations shall he corrected or taxes abated. Equitable remedies shall be resorted to only where the aggrieved party has no taxable property within the tax district of which complaint is made. (Emphasis added.)
The owners responded by arguing that the pre-trial agreement was merely procedural (analogous to waiving jury trial) and that the county assessor is either bound by the agreement or estopped from denying its effectiveness.
¶ 8 The Court of Civil Appeals, in a 2-1 decision, agreed with the county assessor, holding that “[b]y express language, the Legislature has chosen to deny jurisdiction to the District Court to hear such matters in the absence of compliance with these procedures.” The appellate court also took note of an issue raised by the dissent — whether the self-executing constitutional provision upon which the claimed exemption is based frees [111]*111the owners from the effects of the remedial restrictions prescribed by § 2885(A). Because this “constitutional” issue was not raised in the trial court or on appeal, the appellate court declined to address it.
¶ 9 The owners sought certiorari, arguing that the statutory provisions to which the appellate court referred in its opinion, 68 O.S.1991 §§ 2876, 2877, and 2880.1, do not apply to assessments upon constitutionally exempt property. Those sections are said to address only valuations of real and personal property and the addition or assessment of personal property not listed by the taxpayer. Next, the owners argue that the terms of both § 2885(A) and § 2886 “provide a more appropriate alternative basis for jurisdiction in the District Court.” Their final contention is identical to the view expressed by the dissent.
¶ 10 The first issue raised by the owners is whether the statutorily prescribed avenues of complaint before the county assessor and the board of equalization (and appeals therefrom) are applicable to their claim. They are correct in their observation that the language of the statutory remedies to which they refer, in §§ 2876, 2877, and 2880.1 (quoted in part in notes 3, 4, and 5, respectively), indicates a primary concern with changes (or increases) in valuations of real property, as opposed to changes in taxable status. We note, however, that the owners themselves, in response to the county assessor’s notice, asserted their exemption and sought a hearing. The county assessor, who is charged with the duty of preparing the tax rolls and listing exempt property, 68 O.S.1991 § 2868(D), ruled against them. They then appealed to the county board of equalization, which is required to “cancel assessments of property not taxable.” 68 O.S.1991 § 2863(B)(3). No appeal was brought from the board’s adverse decision pursuant to § 2880.1.
¶ 11 Appealing from the board’s ruling, then, was an option for the owners. They could have also complained to the board of tax roll corrections under § 2871(C)(2), the terms of which explicitly authorize the board to determine whether “[pjroperty exempt from taxation has been assessed....” The right of the taxpayer and the county assessor to appeal from decisions of this board to the district court is authorized by § 2871(F).
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LAVENDER, Justice.
¶ 1 The dispositive issue is whether the district court has jurisdiction to grant declaratory relief to a taxpayer who, without first invoking any judicial remedy provided in the Ad Valorem Tax Code, commences an action, challenging the county assessor’s refusal to recognize an exemption claimed under a self-executing provision of the Oklahoma Constitution. We answer in the negative and hold that the legislatively prescribed judicial remedies afforded to these taxpayers under the Code are adequate and exclusive, and that the district court therefore lacked subject matter jurisdiction to entertain their declaratory judgment action.
¶ 2 Owners of an allegedly nonprofit housing project, providing subsidized and minimally-priced housing for elderly and handicapped people, were notified in 1994 by the County Assessor of Muskogee County, Jackie Scott, that their property would no [109]*109longer be listed on the tax roEs as exempt.1 They had acquired the property during the previous year and aEegedly continued to use it as the previous owner did for “charitable purposes” within the meaning of 68 O.S.1991 § 2887(9).2
¶3 In response to the assessor’s notice the owners lodged a complaint pursuant to 68 O.S.1991 § 2876(D),3 which gives taxpayers 20 days to object in writing to an action taken by the county assessor. The latter held a hearing and denied the owners’ protest. They then appealed to the county board of equalization, which also ruled against them.4 The terms of 68 O.S.1991 § 2880.1 give taxpayers and county assessors the right to appeal to the district court from an order of the county board of equalization ■within 10 days after final adjournment of the board.5 In eases where “the illegality of the tax is alleged to arise by reason of some action from, which the laws provide no appeal, the aggrieved person shall pay the full amount of the taxes at the time and in the manner provided by láw, and shall give notice to the officer collecting the taxes showing the grounds of complaint and that suit will be brought against the officer for recovery of them.” (Emphasis added.) 68 O.S. 1991 § 2886(A).
¶4 The owners complied with neither § 2880.1 nor § 2886. Instead, in October 1994, several months after the board’s decision, they sought declaratory relief in the district court, aHeging that the county assessor, “through [the] Muskogee County Board of Equalization,” refuses to recognize the exempt status of their property, despite its [110]*110nonprofit, charitable use. In May 1995 the trial court approved a partial compromise reached by the parties, whereby the owners agreed to pay the 1994 ad valorem tax without admitting to its validity, and the controversy would be converted into an appeal from a hypothetical adverse ruling by the board of equalization regarding the 1995 assessment.6
¶ 5 A hearing was eventually held in August 1995 to determine whether the property is exempt. Before evidence was introduced, however, counsel for the county assessor objected to the trial court’s exercise of jurisdiction and argued that the county assessor had never agreed to the terms of the compromise that had earlier been approved by the court. The trial judge rejected counsel’s assertions and allowed the owners to proceed.
¶6 At the hearing’s conclusion the trial court declared the property exempt, finding that it is “used exclusively for charitable purposes and that the facilities operated by Plaintiffs are open to any person regardless of ability to pay.” The court also found “no evidence that the owners received any remuneration.’.’ The county assessor appealed.
¶ 7 Only one issue was raised on appeal— whether the trial court had jurisdiction to resolve the exemption issue. The basis for asserting lack of jurisdiction was the owners’ failure to appeal to the district court under § 2880.1.7 In support of the exclusiveness of this statutorily prescribed judicial remedy the county assessor relied solely on the terms of 68 O.S.1991 § 2885(A):
A. The proceedings before the county assessor, boards of equalization and appeals therefrom shall he the sole method by which assessments or equalizations shall he corrected or taxes abated. Equitable remedies shall be resorted to only where the aggrieved party has no taxable property within the tax district of which complaint is made. (Emphasis added.)
The owners responded by arguing that the pre-trial agreement was merely procedural (analogous to waiving jury trial) and that the county assessor is either bound by the agreement or estopped from denying its effectiveness.
¶ 8 The Court of Civil Appeals, in a 2-1 decision, agreed with the county assessor, holding that “[b]y express language, the Legislature has chosen to deny jurisdiction to the District Court to hear such matters in the absence of compliance with these procedures.” The appellate court also took note of an issue raised by the dissent — whether the self-executing constitutional provision upon which the claimed exemption is based frees [111]*111the owners from the effects of the remedial restrictions prescribed by § 2885(A). Because this “constitutional” issue was not raised in the trial court or on appeal, the appellate court declined to address it.
¶ 9 The owners sought certiorari, arguing that the statutory provisions to which the appellate court referred in its opinion, 68 O.S.1991 §§ 2876, 2877, and 2880.1, do not apply to assessments upon constitutionally exempt property. Those sections are said to address only valuations of real and personal property and the addition or assessment of personal property not listed by the taxpayer. Next, the owners argue that the terms of both § 2885(A) and § 2886 “provide a more appropriate alternative basis for jurisdiction in the District Court.” Their final contention is identical to the view expressed by the dissent.
¶ 10 The first issue raised by the owners is whether the statutorily prescribed avenues of complaint before the county assessor and the board of equalization (and appeals therefrom) are applicable to their claim. They are correct in their observation that the language of the statutory remedies to which they refer, in §§ 2876, 2877, and 2880.1 (quoted in part in notes 3, 4, and 5, respectively), indicates a primary concern with changes (or increases) in valuations of real property, as opposed to changes in taxable status. We note, however, that the owners themselves, in response to the county assessor’s notice, asserted their exemption and sought a hearing. The county assessor, who is charged with the duty of preparing the tax rolls and listing exempt property, 68 O.S.1991 § 2868(D), ruled against them. They then appealed to the county board of equalization, which is required to “cancel assessments of property not taxable.” 68 O.S.1991 § 2863(B)(3). No appeal was brought from the board’s adverse decision pursuant to § 2880.1.
¶ 11 Appealing from the board’s ruling, then, was an option for the owners. They could have also complained to the board of tax roll corrections under § 2871(C)(2), the terms of which explicitly authorize the board to determine whether “[pjroperty exempt from taxation has been assessed....” The right of the taxpayer and the county assessor to appeal from decisions of this board to the district court is authorized by § 2871(F).8 For reasons explained infra, the owners need not have complied with these statutory remedies before seeking judicial relief.
¶ 12 With respect to § 2885(A) the owners rely on its reference to “[ejquita-ble remedies ... where the aggrieved party has no taxable property within the tax district. ...” They maintain that they qualify for equitable relief because the trial court declared their property to be exempt and therefore nontaxable. We reject this argument for three reasons. The first is that the owners did not seek equitable relief from the trial judge, and none was granted. Secondly, the “equitable remedies” to which the owners refer under § 2885(A) are affordable when the aggrieved party has no taxable property situated within the tax district, and even then, equity will not, for example, reverse the effects of a voluntary payment of taxes under a mutual mistake of fact, where the taxpayer could have earlier discovered the error and sought its correction. Edwards v. Board of Com’rs, 169 Okla. 87, 36 P.2d 6 (1934) (the Court’s syllabus). Finally, a judge’s powers in equity are not invocable when clear and adequate statutory remedies are available. Whig Syndicate, Inc. v. Keyes, 1992 OK 95, 836 P.2d 1283, 1288; Billingsley v. North, 298 P.2d 418, 421-422 (Okla.1956). We now address the main issue in this cause.
¶ 13 The “constitutionality of § 2885(A),” as the appellate court phrased the issue, is not at stake. The prime question is one of subject matter jurisdiction, which can be raised sua sponte, and this Court is duty-bound to inquire into the jurisdiction of the court from whence the decision came. Lincoln Bank and Trust Co. v. Okla. Tax Com’n, 1992 OK 22, 827 P.2d 1314, 1318. Even if the constitutional issue were before us, our duty to address it would not depend on the public interest or welfare, because the [112]*112underlying question is jurisdictional.9
¶ 14 Although codified in § 2887 (see note 2, supra), the exemption claimed by the owners is mandated by the Oklahoma Constitution, Art. 10 § 6:
(a) Except as otherwise provided in subsection (b) of this section, all property used for free public libraries, free museums, public cemeteries, property used exclusively for nonprofit schools and colleges, and all property used exclusively for religious and charitable purposes ... shall be exempt from taxation_ (Emphasis added.)
This Court has long held this provision to be self-executing. See Cox v. Dillingham, 199 Okla. 161, 184 P.2d 976, 980 (1947); City of Hartshorne v. Dickinson, 207 Okla. 305, 249 P.2d 422, 424 (1952); As such it is “operative without supplemental or enabling legislation,” Independent School Dist. No. 9 v. Glass, 1982 OK 2, 639 P.2d. 1233, 1238, and its object is thus placed beyond the power of the legislature to nullify by failing to enact laws to carry it into effect. Wren v. Dixon, 40 Nev. 170, 161 P. 722, 730 (1916).
¶ 15 A tax upon property exempt from taxation by virtue of a self-executing constitutional provision is “illegal and void.” Cox, supra; City of Hartshorne, supra; Glass, supra. Application of this principle in Cox produced the following holding, which might appear at first glance to support the owners’ quest for declaratory relief: “If, by reason of the constitutional exemption, property is not taxable county officials have no jurisdiction or power to levy a tax against the same, and any levy so made is void. If the levy is void the statutory proceeding by appeal from the Board of Equalization is not exclusive.” (Emphasis added.) 184 P.2d at 977 (the Court’s syllabus ¶ 3). A closer look at this ease, however, reveals that it provides no support for the owners’ position.
¶ 16 In Cox the taxpayer, without appealing from the board of equalization’s ruling, sued the county treasurer to recover taxes paid under protest on property claimed to be exempt under Art. 10 § 6. From an order dismissing the action for want of jurisdiction the taxpayer appealed. This Court reversed, rejecting the treasurer’s contention that the taxpayer had an adequate remedy under 68 O.S.1941 §§ 15.41 and 15.49, whose provisions are now found, in substantial part, in §§ 2876 and 2885, respectively. The self-executing constitutional provision freed the taxpayer from the requirements of statutory procedures intended to be followed in asserting the exemption. This Court thus held that an assessment-invalid under Art. 10 § 6 — is not made valid by a taxpayer’s failure to appeal from a ruling of either the county assessor or the board of equalization. An aggrieved party claiming the constitutional exemption may, as the taxpayer did in Cox, timely pay the tax and sue for a refund.10 Cox therefore does not stand for the proposition that the legislature is prohibited from enacting judicial remedies for challenging an allegedly illegal, void, or unconstitutional assessment.
¶ 17 The record in the instant cause is devoid of any indication, and the owners do not contend, that § 2886, for example, would not have provided them a clear and adequate remedy. Self-executing constitutional provisions generally do not preclude the legislature from prescribing judicial remedies. Indeed, Oklahoma’s statutory procedure for seeking judicial relief from an illegal tax has, [113]*113since its initial enactment in 1915,11 been upheld as a plain, speedy, adequate, and exclusive remedy within the legislative power to prescribe. See Black v. Geissler, 58 Okla. 335, 159 P. 1124, 1126 (1916); Atchison, T. & S.F. Ry. Co. v. Eldredge, 65 Okla. 317, 166 P. 1085, 1087 (1917); Parkinson v. Valente, 326 P.2d 826, 828 (Okla.1958); United Airlines, Inc. v. State Bd. of Equalization, 1990 OK 29, 789 P.2d 1305, 1308.12
¶ 18 The instant controversy presents no occasion for this Court to create an alternative remedy. See R.R. Tway, Inc. v. Oklahoma Tax Commission, 1995 OK 129, 910 P.2d 972, 979, where we refused to refashion a judicial tax-refund remedy, because the taxpayer failed to avail himself of his statutory (or administrative) remedies. This is not a case in which no statutory appeal process was available (see Cantrell v. Sanders, 1980 OK 43, 610 P.2d 227, 229), nor is it one in which declaratory and injunctive relief was sought after paying an allegedly illegal tax under protest pursuant to § 2⅛69 (currently § 2886) (see United Airlines, Inc. v. State Bd. of Equalization, supra). Here, the owners were faced with an allegedly illegal tax, and they neither appealed from the board’s adverse ruling under § 2880.1 nor paid the tax under protest and sued for a refund pursuant to § 2886. The district court therefore lacked jurisdiction to grant them declaratory relief.13
¶ 19 Today’s pronouncement is consistent with our recent holdings in Stallings v. Oklahoma Tax Commission, 1994 OK 99, 880 P.2d 912, 918, and Glass, supra. In Stall-ings the taxpayer instituted an action to recover taxes paid but allegedly collected in violation of federal constitutional and statutory law. We held that the district court lacked subject matter jurisdiction because the taxpayer failed to satisfy the statutory prerequisites for invoking the district court’s cognizance.
¶20 In Glass the taxpayer brought a complaint before the board of tax roll corrections and prevailed in his quest for a refund based on the self-executing constitutional exemption in Art. 10 § 6A (for “goods, wares and merchandise” used for manufacturing [114]*114within nine months of shipment to the taxpayer). The school district affected by the decision sued the board and the taxpayer, seeking to enjoin payment of the refund. We rejected the district’s contention that the taxpayer should have first complained to the board of equalization and held that the assessed tax was “illegal and void” and that “[vjoluntary payment by the taxpayer of an invalid tax does not constitute a waiver or ratify the tax if a timely application for a refund is filed in accordance with 68 O.S.Supp.1974 § 24.79 [now § 2871].” (Emphasis added and footnote omitted.) 639 P.2d at 1238. We further concluded that “[a] statute of limitations may be imposed on an action to recover the tax paid, but failure to prove the nature and use of the property before the assessment roll is formulated does not validate an unauthorized and illegal tax.” (Footnote omitted.) 639 P.2d at 1239.14 See also Redbird v. Oklahoma Tax Commission, 1997 OK 126, 947 P.2d 525, 529, where a state statute of limitations barred the taxpayers’ refund claim founded on a federal exemption of Indian land from taxation, and California v. Grace Brethren Church, 457 U.S. 393, 417-418, 102 S.Ct. 2498, 2512-2513, 73 L.Ed.2d 93 (1982), where state law afforded the taxpayer a “plain, speedy and efficient” remedy and thereby deprived the federal district court of jurisdiction to grant injunctive and declaratory relief from enforcement of an allegedly unconstitutional tax.
¶ 21 For the foregoing reasons, the Court of Civil Appeals’ memorandum opinion is VACATED, the trial court’s judgment is REVERSED, and the cause is REMANDED WITH DIRECTIONS TO DISMISS FOR WANT OF SUBJECT MATTER JURISDICTION.
¶ 22 KAUGER, C. J., and HODGES, HARGRAVE, ALMA WILSON and WATT, JJ., concur.
¶ 23 SIMMS and OPALA, JJ., concur in result.
¶ 24 SUMMERS, V.C.J., not voting.