Thanks Jeff - OK I've had chance to have a look at this now and to be honest I'm still not 100%!

Is this how you would multiply out this problem?

LINK TO IMAGE
On of the things that I'm unsure about is the double parentheses in the second bracket set ( where -xy would be in the simpler example)

I'm also unsure how you went from the second line to the third line in the example you gave - is that just multiplying out or was there simplification in that as well?

I'm going to carry on working on it, just thought I'd post that.