There are 2 ways to do this.
1) First you need to factor x² - 9.
x² - 9 = (x + 3 )(x - 3)
So x = -3, 3.
2) Let x² - 9 = 0
So then add 9 to both sides and you'll get:
x² = 9
Square root both sides and you'll get:
x = ± 3 (Which is the same thing for -3, 3)