How can you factor y=x^2-4x-5?

Question

 I forgot how to factor equations like this. Any tips?

Answer 1

Using FOIL,

   (x+a)(x+b) = x^2 + bx + ax + ab

                 = x^2 + (a+b)x + ab

 

In this problem, we are looking for two numbers whose product is -5 and whose sum is -4.  Then numbers +1 and -5 do nicely.

    y=x^2-4x-5 = (x+1)(x-5)

Answer 2

x^2-4x-5

-5*1=-5 and -5+1=-4

(x-5)(x+1)