Iwant 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