(a) The mass of water displaced by the rock is determined by the difference in the rock's mass in air and its apparent mass in water. So, the mass of the water displaced is 540g - 342g = 198g. (b) The volume of the rock can be determined from the mass of the water displaced. Since 1g of water has a volume of 1cm³, the volume of 198g of water (and therefore the volume of the rock) is 198 cm³. (c) The average density of the rock can be calculated using its mass and volume. So, the density is mass/volume = 540g/198cm³ = 2.73 g/cm³.
