referencing multi-line equations

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each line gets its own number. to label a specific line put a label at the end of the line:
\begin{equation}
(a + b)^2 = a^2 + 2ab + b^2 \\
= b^2 + 2ba + a^2 \\
= (b + a)^2\label{eq:3} \\
\end{equation}