\documentclass[12pt,a4paper,openany]{book}
\usepackage{amsmath,amssymb,amsthm}
\usepackage{graphicx}
\usepackage{polyglossia}
\setmainlanguage[numerals=maghrib]{arabic}
\setotherlanguage{english}
\newfontfamily\arabicfont[Script=Arabic,Scale=1.1,AutoFakeSlant=-0.03]{Amiri}
\setsansfont[Script=Arabic,Scale=1.1]{Amiri}
\begin{document}
\begin{equation}\label{n}{\tag{\reflectbox{ن}}}
a=b
\end{equation}
\end{document}