\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
    P(s) = \frac{b(s)}{a(s)} = \frac{B(s)}{A(s)}.
\end{equation*}
\end{document}