Answer
$\dfrac{a}{b^2}$
Work Step by Step
Using the Negative Exponent Rule of the laws of exponents which states that $x^{-m}=\dfrac{1}{x^m}$ or $\dfrac{1}{x^{-m}}=x^m,$ the given expression is equivalent to
\begin{array}{l}\require{cancel}
ab^{-2}
\\\\=
\dfrac{a}{b^2}
.\end{array}