
\documentclass{article}
\usepackage[body={8cm,8cm}]{geometry}
\usepackage{mathtools}
\usepackage{lmodern}
\pagestyle{empty}
\begin{document}
\[ (S) \left\{ % ici on ouvre... 
\begin{aligned} 
  ax + by + cz &= 0 \\
  ey + fz &= 0 \\ 
  gz &= 0 
\end{aligned} 
\right. % ...et là on ferme !
\]
\end{document}
