\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  C_o(s) = -k_o \frac{s+a_o}{s+b_o},
\end{equation*}
\end{document}