WebWork is sometimes very specific about the formatting of answers. This is very dependent upon how your instructor programmed the question (actually, it is more based on how the instructor programmed acceptable answers). Do you have any instructions for the format or syntax it expects? It is possible that you are entering a correct derivative, but using wrong syntax. Would you copy/paste exactly what you are entering into WebWork? Also, please add any instructions for formatting answers.
Edit: I didn't see the previous post where the OP computed the answer.