\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  x_{e}=\frac{\lambda}{\mu_\text{max}-\lambda}.
\end{equation*}
\end{document}