Consider $16a^{4}-25a^{2}b^{2}$. Factor out $a^{2}$.
$$a^{2}\left(16a^{2}-25b^{2}\right)$$
Consider $16a^{2}-25b^{2}$. Rewrite $16a^{2}-25b^{2}$ as $\left(4a\right)^{2}-\left(5b\right)^{2}$. The difference of squares can be factored using the rule: $p^{2}-q^{2}=\left(p-q\right)\left(p+q\right)$.
