Consider $15x^{2}-16x-60+5x^{2}+16x+40$. Multiply and combine like terms.
$$20x^{2}-20$$
Consider $20x^{2}-20$. Factor out $20$.
$$20\left(x^{2}-1\right)$$
Consider $x^{2}-1$. Rewrite $x^{2}-1$ as $x^{2}-1^{2}$. The difference of squares can be factored using the rule: $a^{2}-b^{2}=\left(a-b\right)\left(a+b\right)$.
