\documentclass{article}
\usepackage{xepersian}
\settextfont{XB Niloofar}
\begin{document}
\[
      \begin{array}{lc}
       	\frac{dQ}{d t}=A(\epsilon H - R)\\
       	R=\epsilon(T)R_{bb}(T)\\
       	H=R_{bb}(T_w)
      \end{array}
      \rightarrow \frac{dQ}{d t}=A\epsilon(T)(R_{bb}(T_w)-R_{bb}(T)) \]
\end{document}