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

$\dfrac{9}{x^{8}}$
Using the laws of exponents, the given expression, $(3x^{-4})^{2}$, is equivalent to \begin{array}{l} 3^{2}x^{-4(2)} \\\\= 9x^{-8} \\\\= \dfrac{9}{x^{8}} .\end{array}