\documentclass[12pt]{article}
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\usepackage{amsmath}
\begin{document}
\huge{
$\vec{p}_{electron}\cdot\vec{p}_{electron} = ( \vec{p}_{1}\cdot\vec{p}_{1})+( \vec{p}_{2}\cdot\vec{p}_{2})-2( \vec{p}_{1}\cdot\vec{p}_{2})$
}
\end{document}
