Chapter 5 - Polynomials and Factoring - 5.6 Factoring: A General Strategy - 5.6 Exercise Set - Page 344: 50

$\left( \dfrac{1}{6}a+m \right)\left( \dfrac{1}{6}a-m \right)$

Using $a^2-b^2=(a+b)(a-b)$ or the factoring of the difference of $2$ squares, then the factored form of the given expression, $ \dfrac{1}{36}a^2-m^2 $, is \begin{array}{l} \left( \dfrac{1}{6}a+m \right)\left( \dfrac{1}{6}a-m \right) .\end{array}