%% 
%%  Ein Beispiel der DANTE-Edition
%%  Mathematiksatz mit LaTeX
%%  3. Auflage
%%  Beispiel 05-01-2 auf Seite 78.
%%  Copyright (C) 2018 Herbert Voss
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%%  See http://www.latex-project.org/lppl.txt for details.
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\documentclass[10pt]{screxa}
\pagestyle{empty}
\setlength\textwidth{352.81416pt}
\setlength\parindent{0pt}
%StartShownPreambleCommands
\usepackage{amsmath,xcolor}
%StopShownPreambleCommands
\begin{document}
\begin{align*}
y &= 2x^2 -3x +5\\\nonumber
  & \hphantom{= \ 2\left(x^2-\frac{3}{2}\,x\right. }%
      \textcolor{blue}{\overbrace{\hphantom{+\left(\frac{3}{4}\right)^2- %
        \left(\frac{3}{4}\right)^2}}^{=0}}\\[-11pt]
  &= 2\left(\textcolor{red}{%
     \underbrace{x^2-\frac{3}{2}\,x + \left(\frac{3}{4}\right)^2}}%
     \underbrace{-\left(\frac{3}{4}\right)^2 + \frac{5}{2}}\right)\\
  &= 2\left(\qquad\textcolor{red}{\left(x-\frac{3}{4}\right)^2}
     \qquad + \ \frac{31}{16}\qquad\right)\\
y\,\textcolor{blue}{\mathrel{-}\frac{31}{8}}
  &= 2\left(x\,\textcolor{cyan}{\mathrel{-}\frac{3}{4}}\right)^2%
\end{align*}
\end{document}