Assuming this is not the only such display in your document, you might use `eqparbox`

facilities.

```
\documentclass{article}
\usepackage{enumitem}
\usepackage{eqparbox}
\newcounter{mathenum}
\newenvironment{mathenum}
{\begin{enumerate}\stepcounter{mathenum}}
{\end{enumerate}}
\newcommand{\mathenumitem}[3]{%
\item
\eqmakebox[@@\themathenum @[email protected]][l]{#1}%
${}\Longrightarrow{}$%
\eqmakebox[@@\themathenum @[email protected]][l]{#2}%
\quad#3%
}
\begin{document}
\begin{mathenum}
\mathenumitem{$X$ integrable}
{$E[X|\mathcal{A}_1]$ integrable y $E[E[X|\mathcal{A}_1]]=E[X]$}
{}
\mathenumitem{$\mathcal{A}_1=\{\emptyset,\Omega\}$}
{$E[X|\mathcal{A}_1](\omega)=E[X],\ \forall \omega\in\Omega$}
{}
\mathenumitem{$\mathcal{A}_1=\mathcal{A}$}
{$E[X|\mathcal{A}_1]=X$}
{c.s.$[P_1]$}
\end{mathenum}
\end{document}
```

A slightly different version where the central symbol (which is supposed to be an arrow) can vary:

```
\documentclass{article}
\usepackage{enumitem}
\usepackage{eqparbox}
\newcounter{mathenum}
\newenvironment{mathenum}
{\begin{enumerate}\stepcounter{mathenum}}
{\end{enumerate}}
\newcommand{\mathenumitem}[4]{%
\item
\eqmakebox[@@\themathenum @[email protected]][l]{#1}%
\eqmakebox[@@\themathenum @[email protected]][c]{${}#2{}$}%
\eqmakebox[@@\themathenum @[email protected]][l]{#3}%
\quad#4%
}
\begin{document}
\begin{mathenum}
\mathenumitem{$X$ integrable}
{\Longrightarrow}
{$E[X|\mathcal{A}_1]$ integrable y $E[E[X|\mathcal{A}_1]]=E[X]$}
{}
\mathenumitem{$\mathcal{A}_1=\{\emptyset,\Omega\}$}
{\Longleftrightarrow}
{$E[X|\mathcal{A}_1](\omega)=E[X],\ \forall \omega\in\Omega$}
{}
\mathenumitem{$\mathcal{A}_1=\mathcal{A}$}
{\Longleftarrow}
{$E[X|\mathcal{A}_1]=X$}
{c.s.$[P_1]$}
\end{mathenum}
\end{document}
```

`\Longrightarrow`

. Do be honest, I don't see the point in aligning here.