x^2 - x - 20
(x + 4)(x - 5) <==
x^2 - x - 20.....the last sign, which is negative, tells me that one set is positive and one set is negative. The first sign, which is also negative, tells me that the larger number will be negative. Now u have to find 2 numbers, that when added equal -1 and when multiplied, equal -20....those numbers are -5 and 4. so ur answer is : (x + 4)(x - 5)