Answer
$x=-4$
Work Step by Step
Note that $81=3^4$. Thus, the given equation is equivalent to:
$3^{-x} = 3^4$
Use the rule $a^m=a^n \longrightarrow m = n$ to obtain:
$-x = 4$
Multiply $-1$ to both sides of the equation to obtain:
$x=-4$
