I assume container is filled full. Then volume of water flowing per second = (5700 m^3) / (12 x 60s)
=7.92 m^3 per s
And given density of water is 1g per cm^3 i.e. 1000kg per m^3.
Hence mass flowing per second is = (1000 kg m^-3) x (7.92 m^3 /s)
=7920 kg /s