\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  m_{b}\ddot{q} + b_{s}\dot{q} + k_{s}q = f(t).
\end{equation*}
\end{document}