
\documentclass{article}
\usepackage[body={8cm,8cm}]{geometry}
\usepackage{lmodern}
\usepackage{mathtools}
\pagestyle{empty}

\begin{document}
\begin{equation}
\begin{split}
  a & = b+c-d\\
    & \quad +e-f\\
    & = i
\end{split}  
\end{equation}

\begin{multline}
  a+b+c+d+e+f\\
  +i+j+k+l+m+n
\end{multline}
\end{document}
