The volume of the rocks can be found by calculating the difference in volume between the tank with and without the rocks. Since the water level rises by 2 cm when the rocks are added, the volume of the rocks must be equal to the volume of water displaced by the rocks.
The volume of water displaced can be calculated by finding the volume of the rectangular prism that is formed by the portion of the rocks that is submerged in the water. The length and width of the prism are the same as the base of the tank (30 cm by 90 cm), and the height is 2 cm (since the water level rises by 2 cm when the rocks are added). Therefore, the volume of water displaced by the rocks is:
V_water = 30 cm x 90 cm x 2 cm = 5400 cm^3
To convert the volume to liters, we divide by 1000:
V_water = 5400 cm^3 ÷ 1000 = 5.4 L
Therefore, the volume of the rocks is 5.4 L.