Let the radius of the cone be $$r$$ and height = 21 => Perimeter of base = $$2 pi r = 8$$ => $$r = frac{4}{pi}$$ Volume of cone = $$frac{1}{3} pi r^2 h$$ = $$frac{1}{3} pi (frac{4}{pi})^2 21$$ = $$frac{112}{pi} cm^3$$