Chapter 5 - Section 5.2 - More Work with Exponents and Scientific Notation - Exercise Set - Page 268: 59

$x^{12a+2}$

Using laws of exponents, the given expression simplifies to \begin{align*} & \dfrac{x^{4a}\left( x^{4a} \right)^3}{x^{4a-2}} \\\\&= \dfrac{x^{4a}\left( x^{4a(3)} \right)}{x^{4a-2}} \\\\&= \dfrac{x^{4a}\left( x^{12a} \right)}{x^{4a-2}} \\\\&= \dfrac{x^{4a+12a}}{x^{4a-2}} \\\\&= \dfrac{x^{16a}}{x^{4a-2}} \\\\&= x^{16a-(4a-2)} \\\\&= x^{16a-4a+2} \\\\&= x^{12a+2} .\end{align*}