Answer
The solution set is $\left\{-6, 3\right\}$.
Work Step by Step
Factor the trinomial by looking for the factors of $-18$ whose sum is equal to the coefficient of the middle term ($3$).
These factors are $-3$ and $6$. This means that the factored form of the trinomial is $(n-3)(n+6)$.
Thus,
$n^2+3n-18=0
\\(n-3)(n+6)=0$
Equate each factor to 0; then, solve each equation to obtain:
$n-3=0 \text{ or } n+6=0
\\n=3 \text{ or } n=-6$
The solution set is $\left\{-6, 3\right\}$.
