A sculpture is formed from a square-based pyramid resting on a cuboid. The base of the cuboid and the base of the pyramid are both squares of side 3 cm. The height of the cuboid is 8 cm and the total height of the sculpture is 15 cm. The total mass of the sculpture is 738 g. The cuboid-part of the sculpture is made of iron with density 7.8 g/cm³. The pyramid is made from copper. Calculate the density, in g/cm³, of the copper.