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