Chapter 5 - Section 5.7 - Factoring by Special Products - Exercise Set - Page 310: 64

$\left( \dfrac{a}{2}-\dfrac{b}{7} \right)\left( \dfrac{a}{2}+\dfrac{b}{7} \right) $

Using $a^2-b^2=(a+b)(a-b)$ or the factoring of the sum of 2 squares, then, \begin{array}{l} \dfrac{a^2}{4}-\dfrac{b^2}{49} \\\\= \left( \dfrac{a}{2}-\dfrac{b}{7} \right)\left( \dfrac{a}{2}+\dfrac{b}{7} \right) .\end{array}