\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  s (s + a) + b k_p s + b k_i = s^2 + (a + b k_p) s + b k_i.
\end{equation*}
\end{document}