A sump pump is dumping sand into a pile at a rate of 6 cubic feet per second, forming the shape of a cone. If?

A sump pump is dumping sand into a pile at a rate of 6 cubic feet per second, forming the shape of a cone. If the cone always grows such that the height is three times the radius, find the rate of change of the height of the pile of sand at the point when the height is 12 feet.