\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
\displaystyle e_\text{ss} = \lim_{t\to\infty} (r - y(t)).
\end{equation*}
\end{document}