Chapter 5 - Exponents and Polynomials - Chapter 5 Review Problem Set - Page 231: 72

$-12b$

Using the laws of exponents, the given expression, $ (4ab^{-1})(-3a^{-1}b^2) ,$ simplifies to \begin{array}{l}\require{cancel} 4(-3)a^{1+(-1)}b^{-1+2} \\\\= -12a^{1-1}b^{-1+2} \\\\= -12a^{0}b^{1} \\\\= -12(1)b \\\\= -12b .\end{array}