Question on surface area of cone

A manufacturer intends to use the least amount of material to make a hollow right circular cone of radius r cm and vertical height h cm . The cone must be able to contain 100 of chocolate.

a) Show that the amount of material used or the curved surface area of the cone is

b) Find the radius of the cone so that the amoint of material used will be the minimum. Give your answer in terms of

c) The height of the cone in (b) can be expressed as , where *a* is a constant. Find the value of *a*

I am able to do (a) and (b), but can't do part (c). Can anyone help??