\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\begin{multlined}
first term \\
second term
\end{multlined}
\end{equation}
\end{document}
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{aligned}
A & = B + C\\
& = D + E + F\\
& = G
\end{aligned}
\end{equation}
\end{document}