
\documentclass{article}
\usepackage[body={8cm,8cm}]{geometry}
\usepackage{lmodern}
\usepackage{mathtools}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  \|x\|=
     \begin{cases}
        -x & \text{si } x < 0 \\
        x  & \text{si } x \geq 0
     \end{cases}
\end{equation*}
\end{document}
