Looks right to me. No need to substitute other variables as you did, though. The derivative is , as per the quotient rule, and I would state it that way. The +/- is also somewhat not relevant. Just consider the denominator to be (331 + V) and use an appropriate positive or negative V when you substitute in the values for V.

However, nitpicking aside, those results are correct as far as I can tell.

Grep.

P.S. It would all be more readable if you used LaTex to format your math. It's not too complicated to figure out, and there's a forum where you can learn how to use it. Just a small suggestion.

P.P.S. Oh yeah, noticed a missing parenthesis in your derivative.