Which is a factor of 5x^2-15x-20

When we factor 5x^2-5x-20 you get for an answer:

5 (x-4) (x+1)

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