The volume occupied by the metal cubes cannot be occupied by water. Hence, the maximum amount of water that the tank can hold is lesser than its volume (1000x cm^3). Since the volume occupied by the metal cubes is 2000cm^3, you need to take 1000x - 2000 so that you can find the maximum amount of water the tank can hold when it contains the metal cubes.

Since 30 liters is the maximum amount of water the tank can hold, 1000x - 2000 will be equals to 30 liters, or 30 000 cm^3.