\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
    C_n(s)=\Bigl(\frac{s\sqrt[n]{k}+a}{s+a}\Bigr)^n.
\end{equation*}
\end{document}