%% 
%%  Example 07-07-3 
%%  Copyright (C) 2017 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{328.62788pt}
\setlength\parindent{0pt}
\usepackage[T1]{fontenc}
%StartShownPreambleCommands
\usepackage{amsmath}
\newsavebox\lW
\sbox\lW{$P_{3}+P_{2}-P_{1}$}
%StopShownPreambleCommands
\begin{document}
\begin{itemize}
\item first function \\
  $\displaystyle\makebox[\wd\lW][r]{$P_1$}=\sum_a \in A$
\item but another one \\
  $\makebox[\wd\lW][r]{$\sin\left(P_1\right)$}=blabla$
\item or perhaps \\
  $P_{3}+P_{2}-P_{1}=blablub$
\end{itemize}
\end{document}
