There is not much more that could be done with that expression, as the parenthetical term is not factorable.  It would be helpful to see what answer you are expecting and do realize that some 'textbook answers' are generated independently of publication. Typically, graduate students carefully work out the answers to problems that are supposed to be published. From time-to-time problems are changed, such as when a textbook moves from one revision or another. This means there are multiple sources of potential error. 

Clicking on the 'Options' tab would allow you to see the simplification in greater detail [choose 'a lot of steps'] but there really is nothing more that would be added to the simplification.