\documentclass[12pt]{book}
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\usepackage{xfrac,fontspec,unicode-math}
\usepackage{bm}
\setmathfont[version=cambria]{Cambria Math}
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\begin{document}

\begin{equation}
\nabla _h \cdotp \mathbf{u} ^{n+1} =0
\end{equation}


\begin{equation}
\begin{split}
& \dfrac{\delta q}{\delta t}(\xi _i ^{n+1} , t^{n+1}) := \dfrac{d}{dt}q(Y(t; \xi , t^{n+1}),t)_{\vert \xi = \xi_i^{n+1} , t=t^{n+1}}
\\
& \approx \dfrac{q(Y(t^{n+1}; \xi _i ^{n+1} , t^{n+1}),t^{n+1})-q(Y(t^n; \xi _i ^{n+1} , t^{n+1}),t^n)}{h}
\\
& = \dfrac{q(\xi _i ^{n+1}, t^{n+1})-q(\xi _i ^n,t^n}{h})
\\ & \overrightarrow{v}
\end{split}
\end{equation}

\end{document}