Chapter 6 - Section 6.1 - The Greatest Common Factor and Factoring by Grouping - Exercise Set - Page 427: 54

$-3ab(3ab^2-4)$

Factoring the negative $GCF= -3ab ,$ the given expression, $ -9a^2b^3+12ab ,$ is equivalent to \begin{array}{l}\require{cancel} -3ab \left( \dfrac{-9a^2b^3}{-3ab}+\dfrac{12ab}{-3ab} \right) \\\\= -3ab(3ab^2-4) .\end{array}