\documentclass[preview,border=1pt]{standalone}
\usepackage{amsmath,amssymb,amsthm}


\begin{document}

Consider the incompressible Navier--Stokes equations
\begin{equation}\label{eq:navier-stokes}
\begin{split}
\partial_t u + u\cdot\nabla u + \nabla p &= \Delta u \\
\mathrm{div}(u) &= 0.
\end{split}
\end{equation}
\end{document}