\documentclass{article}
\usepackage{amsmath}
\usepackage{xepersian}

\renewcommand{\theequation}{\thesection.\alph{equation}}
\begin{document}
\setcounter{section}{2}% Just for illustrative purposes
\section{شبکه} \subsection{اقتضایی}
این یک معادله است. 
\begin{equation}
  f(x)=ax \qquad \text{and} \qquad g(x)=bx 
\end{equation}

\begin{equation}
E = mC^2 
\end{equation}
\begin{equation}
E = mC^2 
\end{equation}
\end{document}​