%% 
%%  Example 07-05-1 
%%  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,calc}
%StopShownPreambleCommands
\begin{document}
\[
  z =\;\;\underbrace{%
    \makebox[\widthof{~$x^2+y^2$}][r]{%
      $\sqrt{x^2+y^2}$}}_{=z^2}
\]
\end{document}
