#### Answer

$x=\left\{ -9,-2 \right\}$

#### Work Step by Step

Factoring the given equation, $
n^2+11n+18=0
,$ results to
\begin{array}{l}\require{cancel}
(n+9)(n+2)=0
.\end{array}
Equating each factor to zero (Zero Product Principle), then the solutions to the equation, $
(n+9)(n+2)=0
,$ is
\begin{array}{l}\require{cancel}
n+9=0
\\\\
n=-9
,\\\\\text{OR}\\\\
n+2=0
\\\\
n=-2
.\end{array}
Hence, $
x=\left\{ -9,-2 \right\}
.$