Hey, I'm doing some homework and I'm a little confused with this question.

Find the rate of change of the volume V of a cube with respect to

(a) the length w of a diagonal on one of the faces.

(b) the length z of the one of the diagonals of the cube.

So i'm trying to find

I know that the Volume of a cube is

What i need to do is find a way of expressing V in terms of w.

I know that and solving for x I get . Unless my math is incorrect (which is very possible).

The next step is the plug that into so i get

Do i just find the derivative of that to get answer? I know that's wrong somewhere since the answer is

Can anyone tell me what i'm doing wrong? thank you very much