Answer
$\approx-3.907$
Work Step by Step
Using The Change-of-Base Formula $\log_a{b}=\dfrac{\log_c{b}}{\log_c{a}}$ and then evaluating using a calculator gives
$\log_{\frac{1}{2}}15=\dfrac{\log15}{\log\frac{1}{2}}\approx-3.907$
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