Answer
$5(a+5)(a-5)$
Work Step by Step
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}