\documentclass[12pt,russian]{article}
\usepackage[dvips]{graphics,graphicx}
\usepackage{amsmath,rotate}
\usepackage{epsfig}
\usepackage[english,russian]{babel}
\begin{document}
\begin{equation}
\begin{cases}
{{dy_1} \over {dt}}=(1-y_2^2)*y_1-y_2 \\
{{dy_2} \over {dt}}=y_1
\end{cases}
\eqno(2)
\end{equation}
\end{document}
