\documentclass{article}
\usepackage{graphicx}
\usepackage{amsmath}
\usepackage{xepersian}
\settextfont{Yas}
\setdigitfont{Yas}
\begin{document}
\begin{align*}
S = \left\{
\begin{array}{l}
f_1(x_1,...,x_n)=0 \\
\vdots \\
f_s(x_1,...,x_n)=0 
\end{array}
\end{align*}
\end{document}