Answer
$x=-3$
Work Step by Step
$4(3^{-2x-4})=36$
$\implies 3^{-2x-4}=\frac{36}{4}=9$
$\implies 3^{-2x-4}=3^{2}$
For the above equality to hold, the exponents must be equal.
$\implies -2x-4=2$
$\implies -2x=2+4$
$\implies x=\frac{6}{-2}=-3$