Chapter 5 - Section 5.7 - Factoring by Special Products - Exercise Set - Page 309: 15

$\left(\dfrac{1}{3}-2z\right)\left(\dfrac{1}{3}+2z\right)$

Using $a^2-b^2=(a+b)(a-b)$ or the factoring of the difference of two squares, then, \begin{align*} \dfrac{1}{9}-4z^2 \Rightarrow \left(\dfrac{1}{3}-2z\right)\left(\dfrac{1}{3}+2z\right) .\end{align*}