Question 126340
You actually didn't do anything wrong, but you did do this a longer way than necessary.  What I would have done is used 3x^2 as the common denominator, rather than 3x^3.  So I would have multiplied the first numerator by 3 and the second by x.  Then you quickly get the book's answer.  However, your answer of (X^3+6X)/3X^3 simplifies to the book's answer, because you can factor out x/x (you can take an x out of each term), leaving you with (x^2+6)/3x^2.  Nice job, you were so close!