\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  \aligned 
    z[k+1] &= A z[k] + B u[k] \\
    y[k+1] &= C z[k+1]
  \endaligned
\end{equation*}
\end{document}