You could plug it into a calculator to get your solutions or you could factor. Let's factor.
You need to find two numbers that when multiplied get you -18 and when added get you 3.
Let's get factors of -18:
1 × -18
2 × -9
3 × -6
The only factors that can get you that are -3 and 6.
(x-3)(x+6)=0
Set each factor equal to 0 and solve.
x-3=0
x=3
x+6=0
x=-6
Your solutions are 3 and -6.
