\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
\hat x(k+1) = A \hat x(k) + B u(k) + L(y(k) - C \hat x(k))
\end{equation*}
\end{document}