I think this is how you do it...hope it helps
For you to do area it is length times width.
The formula for the volume of a cone, V, is:
V = [1/3] * π * (radius)^2 * height
Here radius = 6 feet / 2 = 3 feet
Height = 15 feet
V = [1/3] π (3 feet)^2 * 15 feet = 45π feet^3
Answer: 45π feet^3