#### Answer

The solution set is $\left\{-11, 6\right\}$.

#### Work Step by Step

Multiply the left side:
$x^2+5x=66$
Subtract 66 to both sides:
$x^2 + 5x - 66 = 66-0
\\x^2+5x-66=0$
Factor the trinomial by looking for factors of $-66$ whose sum is equal to 5. These factors are 11 and $-6$.
$(x+11)(x-6)=0$
Equate each factor to zero, then solve each equation.
$x+11=0 \text{ o } x-6=0
\\x=-11 \text{ or }x=6$