\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  \| z \| = \sqrt{ z_1^2 + z_2^2 + z_3^2}.
\end{equation*}
\end{document}