\documentclass[12pt,a4paper,fleqn, oneside]{article}
 
\usepackage{amssymb}
\usepackage{amsmath}
 
 
\begin{document}
 
\begin{align*} 
            & d(x,a) &\le d(x,y) + d(y,a) \\
\Rightarrow \hspace{0.5cm} & \inf \{ d(x,a) \} &\le \inf \{ d(x,y) + d(y,a) \} \\
            &&\le d(x,y) + \inf \{ d(y,a) \} \\
\Rightarrow \hspace{0.5cm} & d(x,A) &\le d(x,y)+d(y,A)
\end{align*}
 
\end{document}