%% 
%%  Example 07-07-1 
%%  Copyright (C) 2017 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{158.31393pt}
\setlength\parindent{0pt}
\usepackage[T1]{fontenc}
%StartShownPreambleCommands
\usepackage{amsmath}
%StopShownPreambleCommands
\begin{document}
\begin{align*}
     f(x) &= a\\
     g(x) &= x^2-4x\\
f(x)-g(x) &= x^2+x^3+x\\
     g(x) &= x^2+x^3+x^4+x^5+b
\end{align*}
\end{document}
