Answer
$log_{\frac{1}{2}}8=-3$
Work Step by Step
For all positive numbers $a$, where $a\ne1$, and all positive numbers $x$, we know that $y=log_{a}x$ means the same as $x=a^{y}$.
Therefore, $(\frac{1}{2})^{-3}=8$ means the same as $log_{\frac{1}{2}}8=-3$.