How would I work this out?

A jeweller is making a tapered tube to set a diamond. She works out the dimensions of the curved piece of metal she needs based on the diameter and depth of the stone and draws a rough sketch (attached image)

Apply what you know about the area and circumference of circles to work out:
a: the area of the flat curved section shown on the diagram.
b: the diameter and circumference of the top of the finished tube.
c: the diameter and circumference of the bottom of the finished tube.​