Answer
The solution set is $\left\{-3, 3\right\}$.
Work Step by Step
Add $-9$ to both sides to obtain:
$x^2-9=0$
RECALL:
A difference of two square can be factored using the formula:
$x^2-y^2=(x-y)(x+y)$
Factor the difference of two squares to obtain:
$(x-3)(x-3)=0$
Equate each factor to 0, and then solve each equation to obtain:
$x-3=0 \text{ or } x+3 = 0
\\x=3 \text{ or } x =-3$
The solution set is $\left\{-3, 3\right\}$.
