Chapter 5 - Section 5.2 - More Work with Exponents and Scientific Notation - Exercise Set - Page 268: 25

$\dfrac{x^{14}y^{14}}{a^{21}} $

Using laws of exponents, the given expression simpifies to \begin{align*} & \left( \dfrac{x^{-2}y^{-2}}{a^{-3}} \right)^{-7} \\\\&= \dfrac{x^{-2(-7)}y^{-2(-7)}}{a^{-3(-7)}} \\\\&= \dfrac{x^{14}y^{14}}{a^{21}} .\end{align*}