\documentclass{article}
\usepackage{amsmath} 
\usepackage{xepersian}
%\settextfont{XB Niloofar}
\begin{document}
\begin{equation*}
|x| = \left\{
\begin{array}{lr}
-x & \text{اگر} x < 0\\
0 & \text{اگر}  x = 0\\
x & \text{اگر}  x > 0
\end{array} \right.
\end{equation*}
\end{document}