\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
    u=-K\hat x + k_r r = k_1 (r - \hat x_1) - k_2 \hat x_2.
\end{equation*}
\end{document}