Get rid of the extra line in the middle, and it'll work fine.
As for getting the second part below the first part, are you trying to this on MHF? Or in a regular LaTeX environment? If the latter, I'd recommend looking up the align environment from AMS, I believe. There are other multi-line environments, but that'll get you started. If you're trying to do this on MHF, then end one math environment, skip a line, and start a new math environment. That should do it.