I want to replace the keyboard laptop. Following instructions I had found online, I removed the disk drive, hard drive and all screws (including the one under the memory/hard drive compartment cover -- see red dots). Bottom is loose all around, but feels like it's still being held down somewhere central. I know engineers are told to "get a bigger hammer," and I am one, but I'm reluctant to apply that suggestion. What am I missing in order to remove the back?