\documentclass{article}
\usepackage{etoolbox}
\usepackage{enumitem}
\usepackage[colorlinks]{hyperref}
\usepackage{ptext,changepage}
\usepackage{xepersian} 
\newcounter{MyCounter}
\newtoggle{AlreadyInMyItem}

\let\OldItem\item
\def\item{%
    \global\togglefalse{AlreadyInMyItem}%
    \setcounter{MyCounter}{0}%
    \OldItem%
}

\makeatletter
\def\myitem{%
    \OldItem[%
        \iftoggle{AlreadyInMyItem}{}{%
            \refstepcounter{enumi}%
            \global\toggletrue{AlreadyInMyItem}%
        }%
        \arabic{enumi}%
        (\refstepcounter{MyCounter}\alph{MyCounter}).]\hfill%
      \edef\@currentlabel{\arabic{enumi}(\alph{MyCounter})}%\hfill\hspace{3cm}%
}%
\makeatother

\begin{document}
\ptext[1-4]
\begin{adjustwidth}{.5cm}{.5\linewidth}
\begin{enumerate}[align=left]
    \item \hfill
   $ x=yz $
    \item\label{itm1} \hfill
    $y=bx^{2}+cx+d$
    \myitem 
    $y = ax^{3}+bx^{2}+cx+d$
    \myitem\label{myitm3b} 
    $a=b+c$
    \myitem\label{myitm5b} 
   $ x=yz $ 
     \myitem
    $a=b+c$
    \myitem
   $ x=yz $ 
    \myitem
    $a=b+c$
      \myitem
    $a=b+c$
    \myitem
   $ x=yz $ 
    \item \hfill
    $a=b+c$
    \item \hfill
    $a=b+c$
\end{enumerate}
\end{adjustwidth}
در قانون \ref{myitm3b} و \ref{itm1} داریم ...
\end{document}