Answer
$\approx -3.907$
Work Step by Step
RECALL:
The change-of-base formula for logarithms:
$\log_a{b}=\dfrac{\log{b}}{\log{a}}$
Use a calculator and the formula above to obtain:
$\log_{1/2}{15} = \dfrac{\log{15}}{\log{(1/2)}} \approx -3.907$