Answer
$(a-4)(a-5)$
Work Step by Step
The 2 numbers whose product is $ac=
1(20)=20
$ and whose sum is $b=
-9
$ are $\left\{
-4,-5
\right\}.$ Using these two numbers to decompose the middle term of the given trinomial, $
a^2-9a+20
,$ then the factored form is
\begin{array}{l}
a^2-4a-5a+20
\\\\=
(a^2-4a)-(5a-20)
\\\\=
a(a-4)-5(a-4)
\\\\=
(a-4)(a-5)
.\end{array}