It's pretty close to being right! Your function is and, using the product rule and chain rule, you have
, which is correct.
But you appear to be canceling that "-1/3" with the "3" at the end of the formula. You cannot do that because they are not multiplied together. You can cancel the "1/3" with the "3" at the beginning of the fomula: . You can, in addition, factor out: .