Answer
The solution set is $\left\{-5, -4\right\}$.
Work Step by Step
Factor the trinomial by looking for the factors of 20 whose sum is equal to the coefficient of the middle term (9).
These factors are 4 and 5. This means that the factored form of the trinomial is $(x+4)(x+5)$.
Thus,
$x^2+9x+20=0
\\(x+4)(x+5)=0$
Equate each factor to 0 then solve each equation to obtain:
$x+4=0 \text{ or } x+5=0
\\x=-4 \text{ or } x=-5$
The solution set is $\left\{-5, -4\right\}$.
