Answer
The solution set is $\left\{-3, 4\right\}$.
Work Step by Step
Factor the trinomial by looking for the factors of $-12$ whose sum is equal to the coefficient of the middle term ($-1$).
These factors are $-4$ and $3$. This means that the factored form of the trinomial is $(x-4)(x+3)$.
Thus,
$x^2-x-12=0
\\(x-4)(x+3)=0$
Equate each factor to 0; then, solve each equation to obtain:
$x-4=0 \text{ or } x+3=0
\\x=4 \text{ or } x=-3$
The solution set is $\left\{-3, 4\right\}$.
