Answer
$\dfrac{6}{a^{4}b^{9}}$
Work Step by Step
Using the laws of exponents, the given expression, $
(3a^{-5}b^{-7})(2ab^{-2})
$, is equivalent to
\begin{array}{l}
3(2)a^{-5+1}b^{-7+(-2)}
\\\\=
6a^{-4}b^{-9}
\\\\=
\dfrac{6}{a^{4}b^{9}}
.\end{array}