\documentclass[12pt]{report}
\usepackage{blkarray}
\usepackage{amsmath}

\begin{document}

\[
\begin{blockarray}{cccccc}
 & c_2 & c_3 & c_4 & c_5 \\
\begin{block}{c[cccc]c}
  1 & 1 & 1 & 1 & 1 &3 \\
  0 & 1 & 0 & 0 & 1  &3 \\
  0 & 0 & 1 & 0 & 1  &3 \\
  0 & 0 & 0 & 1 & 1 &3 \\
  0 & 0 & 0 & 0 & 1  &3 \\
\end{block}
 & c_2 & c_3 & c_4 & c_5 
\end{blockarray}
 \]




\end{document}