You make x^2 equal 9 so you subtract and get zero. In this case, x = 3 because 3 times itself is 9 and 9-9 =0

x² - 9 = 0 x² + 3x - 3x - 9 = 0 (x - 3)(x + 3) = 0 x - 3 = 0 or x + 3 = 0 + 3 + 3 - 3 - 3 x = 3 x = -3