\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
x_{2j}(t) = e^{\sigma_jt} (-x_{2j-1}(0) \sin \omega_j t + x_{2j}(0) \cos \omega_j t)
\end{equation*}
\end{document}