If you downloaded the expression tree assignment after Friday afternoon, there is an ExponentiateExpression class in it. This was a mistake and you can just delete it. I apologize for the inconvenience.
Also, the course staff is getting alot of emails asking the same question. While we love answering your emails, we don't like answering the same question 160 times. If you have a question, you can feel free to use the discussion boards, that is what they are there for.