Answer
$4(b-5)(b+5)$
Work Step by Step
Factor out $4$ to obtain:
\begin{align*}
4(b^2-25)\\
4(b^2-5^2)
\end{align*}
Recall:
$$m^2-n^2=(m-n)(m+n)$$
Factor the binomial using the formula above with $m=b$ and $n=5$ to obtain:
$$4(b^2-5^2)=4(b-5)(b+5)$$
