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