\documentclass[12pt]{article}
\pagestyle{empty}
\begin{document}

\begin{displaymath}
\underbrace{n_v+n_{Zc}}_{n_T}= 
\underbrace{\tilde{n}+\hat{n}+\tilde{n}_{Zc}}_{\tilde{n}_T}+
\underbrace{(n^1+n_{Zc})}_{n^1_T}-
\underbrace{(\tilde{n}^1+\hat{n}+\tilde{n}_{Zc})}_{\tilde{n}^1_T}
\end{displaymath}

\end{document}


