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

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