Ideally you should create separate equations across multiple lines, however for a very long equation this wouldn't be very sensible.

You therefore have two real options:

• You use a matrix a divide you equation between cells, i.e.

\begin{align*}
x&=a \\
&+x^3 \\
&+y^3 \\
\end{align*}

• You simply insert the double bachslashes at the end of each line you want, (but this is a bit messy) i.e.

x=a \\ +x^3 \\ +y^3