Man, this class is horrible. I just got a 56 on the midterm and it was one of the top scores in the class since the average is a 40. The book is so unhelpful...they give us some general examples and theories and then bombard us with dozens of questions far more advanced than any of the info they give us. Anyway, enough of a rant, onto my question. I am working on a similar problem to this one. I understand how they get the derivative but have no idea how they go from there to the final solution. This is from our solutions manual which gives us the odd-numbered answers. The step I don't understand is followed by ???.