Question

Factor completely a2 - 9a + 20

Answer 1

(a - 5)(a - 4)
You can check, the numbers add up, because -5 - 4 is 9, and 5 * 4 is 20

Answer 2

a^2-9a+20=a^2-5a-4a+20=(a^2-5a)+(-4a+20)
a^2-9a+20=a(a^2/a-5a/a)+(-4)(-4a/(-4)+20/(-4))
a^2-9a+20=a(a-5)-4(a-5)
a^2-9a+20=(a-5)[a(a-5)/(a-5)-4(a-5)/(a-5)]
a^2-9a+20=(a-5)(a-4)