How can I have linebreaks in my long LaTeX equations?

Question

My equation is very long. How do I get it to continue on the next line rather than go off the page?

Answer 1

I used the \begin{matrix}

\begin{equation}
\begin{matrix}
    line_1 \\ 
    line_2 \\ 
    line_3
\end{matrix}
\end{equation}