
\documentclass{article}
\usepackage[body={8cm,8cm}]{geometry}
\usepackage{eqnarray}
\usepackage{lmodern}
\pagestyle{empty}
\begin{document}
Voici quelques exemples :
\begin{equationarray}{ccccc}
  10a & = & 3x  3y & + & 18z  2w \\
   6a & = & 17x    & + &  5z 19w
\end{equationarray}
\end{document}
