A vertically centered equation number on a multline environment

What you want is the multlined environment provided by mathtools:

\documentclass{article}
\usepackage{mathtools}

\begin{document}

\begin{equation}
\begin{multlined}
a+b+c+d+e+f+{}\\
g+h+i+j
\end{multlined}
\end{equation}

\end{document}

Try to use this:

\begin{equation}\label{xx}
\begin{split}
 a&= b+c-d\\
  & \quad +e-f\\
  &= g+h\\
  &= i
\end{split}
\end{equation}`

This answer is really just an addendum to/augmentation of the answer provided by @Thiago on the use of a split environment within an equation environment to get the vertical centering of the equation number.

• Normally, one uses the split environment to align successive lines on a single common point -- frequently, not necessarily, an = sign.

• However, by leaving off the align symbol, &, entirely from two (or more) consecutive lines, the resulting lines will simply be centered, one line after the other.

• Other alignment possibilities exist too. For example, the following code,

  \begin{equation} \begin{split}
  a+b+c+{}&d=\\
  &e+f+g+h
  \end{split} \end{equation}
  

  serves to have the e of the second line be placed exactly below the d of the first line.