3.0x10^5 Joules
Since the water is already at the freezing point, we just need to remove the enthalpy of fusion for water, which is 333.55 J/g. So:
0.90 kg * 1000 g/kg * 333.55 J/g = 300195 J
Rounding to 2 significant figures means we have to remove 3.0x10^5 Joules of heat to freeze the water.
