First, you can divide both sides by -1 so you have a postive x^2
x^2-x-20=0
Now in order to factor, you need 2 numbers that multiply to -20 and add to -1. These two numbers are -5 and +4.
So we can factor it:
(x-5)(x+4)=0
Then the answers are x=5 and x=-4
Hope this helps!
