\documentclass{article}
    \usepackage{blkarray}
    \usepackage{amsmath}
    \begin{document}

    \[ X = \begin{blockarray}{rrr}
    m & n \\
    \begin{block}{[rr]r}
    A & B & m \\
    C & D & n \\
    E & F & n \\
    \end{block}
    \end{blockarray} \]
    \end{document}