Answer
The solution set is $\left\{-9, 9\right\}$.
Work Step by Step
The given equation can be written as:
$n^2-9^2=0$
RECALL:
A difference of two squares can be factored using the formula:
$a^2-b^2=(a-b)(a+b)$
Factor the difference of two squares to obtain:
$(n-9)(n+9)=0$
Equate each factor to 0, and then solve each equation to obtain:
$n-9=0 \text{ or } n+9 = 0
\\n=9 \text{ or } n=-9$
The solution set is $\left\{-9, 9\right\}$.