Verify the divergence theorem for in the solid limited by the paraboloid and the plane .

My attempt : They ask me the check that holds.

I've already calculated the right hand side, which gave me .

I'm stuck on the left side. I know that is a double integral. I think that I must parametrize the surface and calculate . How do I find ? By calculating the gradient of ? I'm not sure at all about this approach though. Can you confirm it?