\documentclass[12pt,oneside]{book}

\usepackage{amsmath}
\usepackage{empheq}
%\usepackage{mathtools}
\usepackage{graphicx}
\usepackage{xepersian}



\settextfont{Yas}
\setlatintextfont{Times New Roman}



\begin{document}

\begin{empheq}[left=\empheqlbrace]{align}
& f(x)=a+b \\
& g(x) = c+d
\end{empheq}





\end{document}