\documentclass{article}
\usepackage[width=17cm,right=2cm,height=26cm,top=2cm]{geometry}
\usepackage{unicode-math}
\usepackage{xepersian}
\settextfont[Scale=1.5]{XB Yas}
\setdigitfont[Scale=1.2]{XB Yas}
\setlatintextfont[Scale=1.5]{Times New Roman}
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\linespread{2}
\begin{document}
$\underbrace{\Psi( \Psi( \dots \Psi(}_{\text{بار}\ k} n)\dots ))$
\end{document}