\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{equation}
\begin{cases} 3x + 5y + z \\ 7x - 2y + 4z \\ -6x + 3y + 2z \end{cases}
\end{equation}

\begin{equation}
\begin{vmatrix}
3x + 5y + z \\ 7x + 2y + 4z \\ 6x + 3y + 2z
\end{vmatrix}\\
\end{equation}


\end{document}