I'd like to suggest that you employ an aligned environment.
\documentclass{article} % or some other suitable document class \usepackage{amsmath} % for 'aligned' environment \begin{document} \begin{enumerate} \item $\displaystyle \tan^{-1}x + \tan^{-1}y = \tfrac{1}{4}\pi \Longleftrightarrow (x+1)(y+1) = 2$. \medskip Proof.\quad $\begin{aligned}[t] % \tan^{-1}x + \tan^{-1}y = \tfrac{1}{4}\pi % &\Longleftrightarrow (x+1)(y+1) = 2 \\ \tan^{-1}x + \tan^{-1}y = \tfrac{1}{4}\pi &\Longleftrightarrow \tan(\tan^{-1}x + \tan^{-1}y) = 1 \\ &\Longleftrightarrow \frac{x+y}{1-xy} = 1 \\ &\Longleftrightarrow (x+1)(y+1) = 2\,. \end{aligned}$ \end{enumerate} \end{document}