Microsoft Excel is WRONG!!! Beware...
Actually, I'm not so sure it's wrong. I think =-(3+6)^2 only seems ambiguous as to whether the negative refers to the (3+6) or the entire expression. If I were after the opposite of the square, I would have the entire expression in parentheses, =-((3+6)^2). I believe the rule for negation is that it applies to the adjacent TERM.
I would be interested in two things: 1.) the MS Excel documentation, and 2.) an opinion from an actual text or mathematician.