% Fichier ex4.4.9-1.tex - Ch. Rolland/O Reilly France
\documentclass{article}
\usepackage[latin1]{inputenc}
\begin{document}
$$\lbrace \begin{array}{l}
            x = 4a^{2} + 5b - c\\
            y = 7a^{2} + b - 3c
          \end{array}$$
\end{document}
