\documentclass[12pt,a4paper]{book}
\usepackage{amsmath,amssymb,amsthm,color}
\usepackage{xepersian}

\settextfont[Scale=1]{Yas}
\setdigitfont[Scale=1]{Yas}
\setlatintextfont{Times New Roman}

\begin{document}
\begin{equation*}
\begin{array}{r}
x+y+3z= ~~5  \\
2x-y+4z= 11  \\
-~y+~z= ~~3.
\end{array}
\end{equation*}

\end{document}