\documentclass[a4paper,11pt]{book}
\usepackage{amssymb,amsmath}
\usepackage[total={13cm,20cm},includehead=true,centering]{geometry}
%\usepackage{fontspec}
\usepackage{pstricks,pstricks-add}
\usepackage[colorlinks,linkcolor=black,citecolor=magenta]{hyperref}
\usepackage{xepersian}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\newenvironment{example} 
{\textcolor{red}{\rule{\textwidth}{3pt}}\\ {\Large$\blacksquare$}}
{\hfill {\Large$\blacksquare$}}



\begin{document}



\rule{\textwidth \black}{3pt}
\rule{\textwidth \gray}{3pt}
\rule{\textwidth \red}{3pt}
%%%

\vspace{4cm}

\begin{example}
\noindent \textbf{مثال}\\
 این یک آزمایش است.
\begin{equation*}
a^2+b^2=c^2
\end{equation*}
\end{example}

\end{document}