也许这种布局更容易让人接受:
\documentclass{article}
\usepackage[a6paper,margin=3cm]{geometry}
\usepackage{mathtools}%% loads amsmath
\begin{document}
\begin{equation}
\begin{alignedat}{2}
a + b & + c & & + d \\[-1ex]\:{}+ e & + f & & + g
\end{alignedat}
=0
\end{equation}
\end{document}