Solve x^2 - 9 = 4x + 12
x2 - 9 = 4x + 12 x2 - 9 - 4x = 12 x2 - 9 - 4x -12 = 0 x2 - 4x -21 = 0 Factorise into form (x + ?)(x + ?) Factors of -21 are +7 and -3 or -7 and +3 (x + 7)(x - 3) = x2 + 7x - 3x -21 = x2 + 4x -21 which isn't what we want so it must be (x - 7)(x + 3)(x - 7)(x + 3) = 0 so x = 7 and x = -3
