Factor completely a2 - 9a + 20

(a - 5) (a - 4)

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

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)