\documentclass[12pt]{article}

\usepackage{amsmath}
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\begin{document}
\[
C'_B\left(G\right) = \frac
    {\sum_{v \in V}{C_B\left(v^{*}\right) - C'_B\left(v\right)}}
    {n - 1}
\]
\end{document}