Functions: Domain of Logarithmic Function

For $f(x)$ to be defined: $\log_3(\log_5(20x - x^2 - 91)) > 0 \implies \log_5(20x - x^2 - 91) > 3^0 = 1 \implies 20x - x^2 - 91 > 5^1 = 5 \implies x^2 - 20x + 96 < 0 \implies (x - 8)(x - 12) < 0 \implies x \in (8, 12)$.