Answer
$\approx-22.0488$
Work Step by Step
Using $\log_a b=\dfrac{\log b}{\log a}$ or the Change-Of-Base Theorem, the given expression, $
\log_{0.91} 8
,$ evaluates to
\begin{array}{l}\require{cancel}
\dfrac{\log 8}{\log 0.91}
\\\\
\approx-22.0488
.\end{array}