maths differential equations question

hi, please could I have some help on part a of this question? So i did V=A*rootA because root A will give one side length. Then got dv/da equals 1.5 root A which is wrong. But for the next part I used this and managed to get to the final answer and I'm not sure where I've gone wrong?
question: https://ibb.co/JRsXDnTJ
Thanks!

Thats for one face/square. There are six faces on a cube.

But I did the cross sectional area of one side (A) multiplied by the length of one more side which will give volume?
Volume is x^3 I just did x^2( which is area)* x which is root area?

For a cube of volume
V = x^3
Then the surface area of the cube is
A = 6 x^2
...

doesn't the final answer have to be in terms of A though?

Yes, you have two equations and you want to eliminate one variable (x) ...
But its better to be clear and write out the original equations explicitly, then do the (simmple) algebra.