Answer
$x=-1$.
Work Step by Step
We know that by definition then $log_a {y}=x$, then $y=a^x$. Hence if $log_2 {(8^x)}=-3$, then $2^{-3}=\frac{1}{8}=8^{-1}=8^x$, therefore because the base is the same on the two sides of the equation, the exponents must be equal too, hence $x=-1$.
