A better way to do it, is the TheHe meantioned, with `amsmath`:

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

    \[
    M=
      \begin{bmatrix}
        1 & 2 & 3 & 4 & 5\        3 & 4 & 5 & 6 & 7\      \end{bmatrix}
    \]

    \end{document}

The `bmatrix` environment will give you `[]` braces. `()` braces are also very common. They are created with the `pmatrix` environment. To include a matrix inline, you can write:

    $M = \left\[ \begin{smallmatrix} 1 & 2 \\ 3 & 4 \end{smallmatrix} \right\]$