Answer
$\dfrac{1}{t^{20}x^{12}}$
Work Step by Step
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}