Step-by-step solving:
Use the method (x - root 1)(x - root 2).
(x - (-3))(x - 6) = 0
(x + 3)(x - 6) = 0
x² - 6x + 3x - 18 = 0
x² - 3x - 18 = 0
When you do it the other way around:
(x - 6)(x - (-3)) = 0
(x - 6)(x + 3) = 0
x² + 3x - 6x - 18 = 0
x² - 3x - 18 = 0
