\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
 u = - k_p (x - x_e) - k_i z + k_r r.
\end{equation*}
\end{document}