
\documentclass{article}
\usepackage[body={8cm,8cm}]{geometry}
\usepackage{lmodern}
\usepackage{delarray}
\pagestyle{empty}
\begin{document}
\[
\|x\| =
\begin{array}\{ {cc}.
   -x & \textrm{si } x < 0 \\
    x & \textrm{si } x \geq 0
\end{array}
\]
\end{document}
