\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
    y[k] = C A^k x[0] + \sum_{j=0}^{k-1} C A^{k-j-1} B u[j] + D u[k].
\end{equation*}
\end{document}