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

$5(a+5)(a-5)$

Factoring the $GCF= 5 $, the given expression, $ 5a^2-125 ,$ is equivalent to \begin{array}{l} 5(a^2-25) .\end{array} Using $x^2-y^2=(x+y)(x-y)$ or the factoring of the difference of 2 squares, then the factored form of the expression above is \begin{array}{l} 5(a+5)(a-5) .\end{array}