\documentclass{article}
\usepackage{graphicx}
\usepackage{amsmath}
\usepackage[subnum]{cases}
\usepackage{xepersian}
\settextfont{Yas}
\setdigitfont{Yas}
\begin{document}
\begin{numcases}
\dot{x} &= ax + by - 3x^2y + y^3, \label{eq1a}  \\ 
\dot{y} &= - \frac{a^2 + 1}{b}x - ay +3xy^2.  \label{eq1b}
\end{numcases}
\end{document}