#### Answer

The solution set is $\left\{-10, 2\right\}$.

#### Work Step by Step

Add $-20$ to both sides of the equation to obtain:
$x^2+8x-20=0$
Factor the trinomial by looking for the factors of $-20$ whose sum is equal to the coefficient of the middle term ($8$).
These factors are $-2$ and $10$. This means that the factored form of the trinomial is $(x-2)(x+10)$.
Thus,
$x^2+8x-20=0
\\(x-2)(x+10)=0$
Equate each factor to 0; then, solve each equation to obtain:
$x-2=0 \text{ or } x+10=0
\\x=2 \text{ or } x=-10$
The solution set is $\left\{-10, 2\right\}$.