V of cylinder = πr²h
∴v of cone = 1/3 πr²h
∴we can take h=7cm , d=12cm, r=6cm
surface area of a cone=πrl when l=slant height of cone.
to find the slant height:
draw a perpendicular from top vertex to the mid of the diameter.
∴slant height = √(7²+6²) = √85 = 9.2
∴surface area of the cone = π * 6 *9.2
= 173.48 cm²