% Fichier ex4.4.9-4.tex - Ch. Rolland/O Reilly France
\documentclass{article}
\usepackage[latin1]{inputenc}
\begin{document}
$\underbrace{
   x = \overline{a + b} 
       - \sqrt{a^{2}+b^{2}}
 }_{Une~\acute equation...}$
\end{document}
