You can put this solution on YOUR website!
They seem to want to have an expression with no denominator, that is all.
Other people would prefer to have an expression with no negative powers, and so would keep the denominator of
. Just depends what the question asked for specifically:
"Bring every term up to the numerator" or "leave all exponents as positives" etc.
If the question didn't specify, then the book authors are just doing what they want to... either version is correct though mathematically.