Don t panic. There are three major reasons for this conflict between your answer and the one in the book. It s impossible to print an error-free math textbook. There are mistakes in the answers. The textbook that we use is better than most, and we have tried mightily to proofread the course guide s answers carefully, but there are still wrong answers in the books.
Space is at a premium in any textbook.
The answer in the book may have been algebraically manipulated to fit onto a single line. Your answer might, in fact, be wrong. Compare your answer with the book s, and use the similarities and differences to focus on the particular spot where you disagree. If you still can t find the error, look at other problems similar to the one you just did. If your answers agree on four out of five problems, you understand how to do the problems and you can safely forget this one.