\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
\begin{aligned}
  \dot x &= A x + B u \\
  y &= C x + D u
\end{aligned}
\end{equation*}
\end{document}