A cone is one third of the volume of the cylinder of the same base. Since a cylinder has a volume of pi x radius2 x height, the radius being 5.25m and the height being 3m, the volume of the corresponding cone would be 259.7704425m3.
The surface area of the top of the cone is defined as pi x radius x slant height. The slant height, by pythagoras, is 6.046693311m, and so the area of the canvas required to cover the heap would be 99.73029825m2.