Do the grouping $x^{3}+5x^{2}-4x-20=\left(x^{3}+5x^{2}\right)+\left(-4x-20\right)$, and factor out $x^{2}$ in the first and $-4$ in the second group.
$$x^{2}\left(x+5\right)-4\left(x+5\right)$$
Factor out common term $x+5$ by using distributive property.
$$\left(x+5\right)\left(x^{2}-4\right)$$
Consider $x^{2}-4$. Rewrite $x^{2}-4$ as $x^{2}-2^{2}$. The difference of squares can be factored using the rule: $a^{2}-b^{2}=\left(a-b\right)\left(a+b\right)$.
