Answer
$-\dfrac{1}{3a^5}$
Work Step by Step
Using the laws of exponents, the given expression, $
\dfrac{-15a^8}{45a^{13}}
,$ simplifies to
\begin{array}{l}\require{cancel}
\dfrac{-\cancel{15}a^{8-13}}{\cancel{15}(3)}
\\\\=
\dfrac{-(a)^{-5}}{3}
\\\\=
\dfrac{-1}{3a^5}
\\\\=
-\dfrac{1}{3a^5}
.\end{array}