Latex matrix not displaying right numbers

Question

I've just started writing in latex to expose math problems to a forum.
I am trying to display a matrix:

\[
\begin{align*}
 \begin{matrix}
  1 & -9 & 3 \\
  1 & 2 & -2 \\
  -2 & 1 & 1
 \end{matrix}
\end{align*}
\]

But I get this:

There's 1b1 and there's not the last element that I'm trying to display, why?

Answer 1

As @Peter Grill mentioned, \[ already starts math mode, so you don't need align*. If you need square brackets as delimiters you can use \left[ and \right] commands or predefined environment bmatrix.

\documentclass[a4paper,12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}

\begin{document}

%
% Option #1
%
\[ \left[
\begin{matrix}
  1 & -9 & 3 \\
  1 & 2 & -2 \\
  -2 & 1 & 1
\end{matrix}
\right] \]

%
% Option #2
%
\[ 
\begin{bmatrix}
  1 & -9 & 3 \\
  1 & 2 & -2 \\
  -2 & 1 & 1 
\end{bmatrix}
\]

\end{document}

Both approaches give the same result: