X^(2)+x-20
splitting the middle term

Are you trying to factorise this?

Here is a way you may not have been taught
(x + a)(x + b) = x^2 + (a + b)x + ab

So you need to find two factors of -20 that add up to +1
-4 x 5 = -20 and -4 + 5 = 1. Perfect

x^2 + x - 20 = (x - 4)(x + 5)