Answer
$\approx-0.1647$
Work Step by Step
Using $\log_a b=\dfrac{\log b}{\log a}$ or the Change-Of-Base Theorem, the given expression, $
\log_8 0.71
,$ evaluates to
\begin{array}{l}\require{cancel}
\dfrac{\log0.71}{\log 8}
\\\\
\approx-0.1647
.\end{array}