\documentclass[addpoints,12pt]{exam}
%\documentclass[a4paper]{article}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage{graphicx}
\usepackage{graphicx}
\usepackage{xepersian}
\settextfont[Scale=1]{Yas}
%\setlatintextfont{Linux Libertine}
\setdigitfont[Scale=1]{Yas}
\setlength{\textwidth}{165mm} % set width of text portion
\setlength{\textheight}{260mm} % set of length 
\setlength\voffset{-15mm} %above margine 
\setlength\hoffset{-5mm} %left margine
%\addtolength{\hoffset}{-20mm} % left margine
\pagestyle{empty} %Page Number just number and for no number using {empty}!
\newcommand{\bc}{$\bigcirc$}
\newcommand{\dlim}{\displaystyle\lim}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{document}

 \begin{equation*}
x=\left\{
\begin{array}{rl}
y\quad 10 \leq x \leq 12\\
y^2\quad  12<x<13,
\end{array}
\end{equation*}
 \begin{equation*}
x^2=\left\{
\begin{array}{rl}
2y\qquad 9 \leq x < 12\\
3y\quad 12<x<15
\right.
\end{array}
\end{equation*}
\end{document}