Answer
$(x+9)(x+2)$
Work Step by Step
The 2 numbers whose product is $ac=
1(18)=18
$ and whose sum is $b=
11
$ are $\left\{
9,2
\right\}.$ Using these two numbers to decompose the middle term of the given trinomial, $
x^2+11x+18
,$ then the factored form is
\begin{array}{l}
x^2+9x+2x+18
\\\\=
(x^2+9x)+(2x+18)
\\\\=
x(x+9)+2(x+9)
\\\\=
(x+9)(x+2)
.\end{array}