Use completing the square to solve (x - 12) (x + 4) = 9 for x. x = - 1 or 15 x = 1 or 7

(x - 12) (x + 4) = 9

x² + 4x - 12x - 48 = 9

x² - 8x - 48 = 9 |add 48 to both sides

x² - 8x = 57

x² - 2x · 4 = 57 |add 4² to both sides

x² - 2x · 4 + 4² = 57 + 4² |use (a - b) ² = a² - 2ab + b²

(x - 4) ² = 57 + 16

(x - 4) ² = 73 ⇔ x - 4 = √73 or x - 4 = - √73 |add 4 to both sides

x = 4 + √73 or x = 4 - √73