\left[
\begin{array}{rrr}
2 & -1 & 1
\vspace{0.5em}\\
-1 & 2 & 1
\vspace{0.5em}\\
1 & 1 & 2
\end{array}
\right]
\left[
\begin{array}{r}
-1
\vspace{0.5em}\\
-5
\vspace{0.5em}\\
0
\end{array}
\right]=
\left[
\begin{array}{r}
3
\vspace{0.5em}\\
-9
\vspace{0.5em}\\
-6
\end{array}
\right].