\documentclass[12pt]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\RequirePackage{amsmath,amssymb,latexsym}
\pagestyle{empty}
\begin{document}
$  \overline{u}=(\alpha_{1} x_{1}+\alpha_{2} y_{1}+\alpha_{3} z_{1})\overline{e}_{1}+ (\alpha_{1} x_{2}+\alpha_{2} y_{2}+\alpha_{3} z_{2})\overline{e}_{2}+(\alpha_{1} x_{3}+\alpha_{2} y_{3}+\alpha_{3} z_{3})\overline{e}_{3} $
\end{document}
