\documentclass{standalone}

\usepackage{amsmath}

\begin{document}
\[
    \begin{Bmatrix}
       a_1 x+b_1 y+c_1 z &=& d_1
       \\
       a_2 x+b_2 y+c_2 z &=& d_2
       \\
       a_3 x+b_3 y+c_3 z &=& d_3
       \\
    \end{Bmatrix}
\]
\end{document}