Hello. I've worked this problem several times, but I keep coming up with the wrong answer and it's driving me mad. Here it is:

The answer is 1/6, but I keep getting undefined. I multiply by the conjugate, then reduce, and I get: $\displaystyle \frac{1}{\sqrt{x+7}-3}$ I have no clue where I'm messing up with the algebra.

Thanks for any help.