Solve x^2 - 9 = 0.

Recognize a difference of squares:
x^2 - 9 = x^2 - 3^2

Factor:
(x - 3)(x + 3) = 0

Set each factor equal to zero:
x - 3 = 0
x + 3 = 0

Therefore:
x = 3 or x = -3