\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  \gathered
  q(t) = \frac{F_0}{k} \left(
     1 - {\color{blue} \frac{1}{\omega_d} \sqrt{\frac{k}{m}}}
     \exp\bigl(-\frac{ct}{2m}\bigr) \sin(\omega_d t + \phi) \right),
  \\[1ex]
  \aligned 
    \omega_d &= \dfrac{\sqrt{4km - c^2}}{2m}, \qquad
    \phi &= \tan^{-1}\left( \frac{\sqrt{4km - c^2}}{\color{blue} c} \right).
  \endaligned
  \endgathered
\end{equation*}
\end{document}