1. Your answer can't be right because a volume has the dimension 3 but your term at the RHS has the dimension 2.

2. The volume of the cone is calculated by:

3. The radius of the piece of paper, the radius of the cone and the height of the cone form a right triangle:

4. Substitute into the equation of the volume. You'll get a function wrt h:

5. Solve for h: V'(h) = 0. I've got

6. Re-substitute to calculate r. I've got

7. Plug in these values to calculate the maximum volume: