\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 + Bu +L(y-C\hat x)=(A-BK-LC)\hat x + 
  B k_r r + L y.
\end{equation*}
\end{document}