Chapter 4 - Polynomials - 4.1 Exponents and Their Properties - 4.1 Exercise Set - Page 236: 76

$\dfrac{16}{a^4}$

Using $\left(\dfrac{a}{b}\right)^x=\dfrac{a^x}{b^x}$, then the expression, $ \left(\dfrac{2}{a}\right)^4 $, simplifies to \begin{array}{l}\require{cancel} \dfrac{2^4}{a^4} \\\\= \dfrac{16}{a^4} .\end{array}