## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$x=-4$
Re-write the given equation as: $3^{-x}=3^4$ $(Because: \ 81=3^4)$ Use the rule power rule: $a^p=a^q$ . We can see that the base $a=3$ is the same on both sides of the equation. So, the exponents will also be equal. This implies that $p=q$ Therefore, $3^{-x}=3^{4} \implies x=-4$