Answer
$(x+11)(x+9)$
Work Step by Step
The 2 numbers whose product is $ac=
1(99)=99
$ and whose sum is $b=
20
$ are $\left\{
11,9
\right\}.$ Using these two numbers to decompose the middle term of the given trinomial, $
x^2+20x+99
,$ then the factored form is
\begin{array}{l}
x^2+11x+9x+99
\\\\=
(x^2+11x)+(9x+99)
\\\\=
x(x+11)+9(x+11)
\\\\=
(x+11)(x+9)
.\end{array}