\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*} \aligned x_{k+1} &= f(x+k, u_k) \\ y_k &= h(x_k, u_k) \endaligned \end{equation*}
\end{document}