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$$