\nonstopmode
\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\pagestyle{empty}
\begin{document}
$$
{\begin{cases}3x+5y+z\\7x-2y+4z\\-6x+3y+2z\end{cases}}
$$
\end{document}
