%\documentclass{report}

%\usepackage{graphicx,amsthm,amsmath,amssymb}
%\usepackage{tikz}

\newtheorem{defe}{تعریف}
\newtheorem{theorem}{قضیه}
\newtheorem{lemma}{لم}
\newtheorem{proposition}{گزاره}
%\usepackage{xepersian}
\begin{document}
\begin{equation}
h_1(x) =\int_{a}^{x}(x-t)g(t)\;dt+\frac{(a-x)}{(b-a)+\lambda(\xi - a)}\left[ \int_{a}^{b}(b-t)g(t)\;dt+\lambda \int_{a}^{\xi}(\xi-t)g(t)\;dt \right] .
\end{equation}

\end{document}