\documentclass{article}
\usepackage{blkarray}
\newcommand{\matindex}[1]{\mbox{\scriptsize#1}}
\begin{document}
\[
x=\begin{blockarray}{ccccccccccccc}
\matindex{$x _{1} $} & \matindex{$x _{2} $} & \matindex{$ x _{3} $} & \matindex{$ x _{4} $} & \matindex{$ x _{5} $} & \matindex{$ x _{6} $} & \matindex{$ x _{7} $} & \matindex{$ x _{8} $} & \matindex{$ x _{9} $} & \matindex{$ x _{10} $} & \matindex{$ x _{11} $} & \matindex{$ x _{12} $} &\\
\begin{block}{(cccccccccccc)c}
1 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 3 & 0 & 0 & \matindex{$ x _{1} $}\\
1 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & \matindex{$ x _{2} $} \\
1 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & \matindex{$ x _{3} $}\\
1 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 3 & 0 & \matindex{$ x _{4} $}\\
1 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & \matindex{$ x _{5} $} \\
1 & 0 & 1 & 0 & 0 & 0 & 0 & 0 7 0 & 0 & 0 & 0 & \matindex{$ x _{6} $}\\
1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 3 & 0 & 0 & 0 & \matindex{$ x _{7} $}\\
1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & \matindex{$ x _{8} $}\\
1 & 0 & 0 & 0 & 0 & 0 & 0 7 .0 & 0 & 0 & 0 & 0 & \matindex{$ x _{9} $}\\
1 & 0 & 0 & 0 & 0 & 0 & 0 7 .0 & 0 & 0 & 0 & 0 & \matindex{$ x _{10} $}\\
1 & 0 & 0 & 0 & 0 & 0 & 0 7 .0 & 0 & 0 & 0 & 0 & \matindex{$ x _{11} $}\\
1 & 0 & 0 & 0 & 0 & 0 & 0 7 .0 & 0 & 0 & 0 & 0 & \matindex{$ x _{12} $}\\
\end{block}
\end{blockarray}
\]
\end{document}
