Pipe a is a inlet pipe filling the tank at 8000l/hr and pipe b is the outlet pipe which empties the tank in 3 hours. the capacity of the tank is

Well , in 3 hours , the amount of the tank that will be filled with pipe A = 8,000 x 3 = 24,000 L

Pipe could empty that 24,000 L in 3 hours.

So the capacity is  around 6 - 8 L .  .. . we need additional information to find out for sure

hope this helps