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

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