Factor the quadratic 9 x^2 + 21 x - 8. The coefficient of x^2 is 9 and the constant term is -8. The product of 9 and -8 is -72. The factors of -72 which sum to 21 are -3 and 24. So 9 x^2 + 21 x - 8 = 9 x^2 + 24 x - 3 x - 8 = 8 (3 x - 1) + 3 x (3 x - 1):
