$ 729-9^{2x-4}=3^{23} \rightarrow \frac{3^23}{3^4}=\frac{729 - 9^{2x-4}}{3^4} \\ 3^19 = \frac{729}{3^4}-\frac{9^{2x-4}}{3^4} = \frac{3^6}{3^4} - \frac{(3^2)^{2x-4}}{3^4} = 3^2 - \frac{3^{4x-8}}{3^4} = 3^2-3^{4x-8-4} = 9-3^{4x-12} = 9-(3^4)^{x-3} = 9-81^{x-3} $