Hurkyl

It looked like a request for an answer, not a request for help -- it is strictly against our policy to hand out answers to homework problems.

My response was meant in all seriousness -- you should have seen in your class a very powerful theorem about homomorphisms from polynomial rings. And sometimes, it's easier to find/work out a solution when you consider the general case without regard to the specific. I assume from your response that you were able to recall the answer? (or able to track it down in your book?)