I want someone to show me how to factor a^2 - a - 20 completely

so what you do is

a^2-a-20

see, the number in front of the a^2 is 1, so now we do

look at the number infront of the a and the number at the end

-1 and - 20

whhat 2 numbers multiply to get - 20 and add to get - 1

trial and error

-20+1=-19, nope

-10+2=-8 nope

-5+4=-1, yes

(a-5) (a+4) is factored