10 litres per minute, thus 3 minutes would mean 30 litres.
30 litres convert to cm cube wouks be 30,000 cm3 .
Volume of each metal cube will be 1000cm3
2 metal cubes will be 2000cm3
Hence to find the volume of the tank,
You need to take 30,000cm3 minus 2 metal cubes = 28,000cm3

Length x breath x height = volume
40 x 25 x height = 28,000cm3
Height = 28cm

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.