Factor the quadratic 15 c^2 + 37 c + 20. The coefficient of c^2 is 15 and the constant term is 20. The product of 15 and 20 is 300. The factors of 300 which sum to 37 are 12 and 25. So 15 c^2 + 37 c + 20 = 15 c^2 + 25 c + 12 c + 20 = 5 (5 c + 4) + 3 c (5 c + 4):
