Oookay.. I know this has to be easy :p but I can't figure it out.

My project drawer on one of my projects has suddenly decided to be on the right side instead of the left. It always used to be on the left. I'm sure I accidentally hit some key combination that did it-- but for the life of me I can't find how to change it back and it's driving me insane :)

Anyone know how to fix? Thanks :)