You made your mistake in the first step, unfortunately. You need to differentiate that as a product rule; you can't just differentiate it as if x were a constant. I'm gonna change things around, so it's a little bit easier. Hopefully this doesn't confuse you...

Differentiating implicitly, you should get

You're trying to isolate , so you can rearrange the equation so it reads as follows:

Solve using the point you're given, and you should get

Hopefully, that is the answer you were looking for?