f(x) = x^2 - 3x - 18
Find the values that when multiplied equal -18 and when added equal -3 (6 and -3)
Plug it into f(x) = (x - 1st value)(x - 2nd value)
f(x) = (x - 6)(x + 3)
0 = (x - 6) (x + 3)
(One of the factors has to equal zero, so set them both equal to zero)
x - 6 = 0
x = 6
x + 3 = 0
x = -3
x = 6, x = -3