A simple way to achieve this is to place each line in the second column of an
aligned environment, which is itself placed inside an equation environment. Then, the whole structure is centred, with each line beginning at the same point. You can create the necessary indentations using
Note that it is possible to use
\hspace here because the space is not the first item on the line (otherwise you would need
\hspace* to prevent the space being discarded). Of course, you will need to adjust the size of the
\hspaces manually, so if you need a lot of similar constructions then a more sophisticated approach may be needed.
Incidentally, I don't think you can use
split inside a
multline (as in your example); if you try you will get the warning
Package amsmath Warning: Cannot use
split' here; (amsmath)aligned' on input line 19.
trying to recover with