5x^2-15x-20= 5(x2-3x-4)= Now, what 2 #'s have a sum of -3 and a product of -4? -4 +1=-3 and (-4)(1), so let's break -3x down to -4x and 1x(or just 'x'): 5(x^2+x-4x-4)= 5{(x^2+x) + (-4x-4)}= Now, over common factors from the terms inside the brackets: 5{x(x+1)-4(x+1)}= Since we now have both x and -4 multiplied by ( x+1), it's the same as: 5(x-4)(x+1) and that is it