Answer
$\dfrac{2b^{3}}{a^{4}}$
Work Step by Step
Using the laws of exponents, the given expression, $
\dfrac{-6a^3b^{-5}}{-3a^7b^{-8}}
$, is equivalent to
\begin{array}{l}\require{cancel}
\dfrac{\cancel{-3}\cdot2a^{3-7}b^{-5-(-8)}}{\cancel{-3}}
\\\\=
2a^{-4}b^{3}
\\\\=
\dfrac{2b^{3}}{a^{4}}
.\end{array}