Chapter 5 - Section 5.7 - Negative Exponents and Scientific Notation - Exercise Set - Page 409: 72

$\dfrac{1}{a^{20}b^{12}}$

Using the laws of exponents, the given expression, $ (a^5b^3)^{-4} ,$ simplifies to \begin{array}{l}\require{cancel} a^{5(-4)}b^{3(-4)} \\\\= a^{-20}b^{-12} \\\\= \dfrac{1}{a^{20}b^{12}} .\end{array}