Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$\dfrac{2b^{3}}{a^{4}}$
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}