First you need the balanced chemical equation:
2 NaHCO3 --> HOH + CO2 + Na2CO3
So if you heat two moles of baking soda, you produce one mole of water, one mole of carbon dioxide and 1 mole of washing soda.
In your example you have 360 kg HOH = 20 kmol; 880 kg CO2 = 20 kmol
You start with 3360 kg of NaHCO3 = 40 kmol
So you will produce 20 kmol of washing soda.
20 kmol * 106 kg/kmol = 2120 kg of washing soda.
A simple solution to this problem relies on the Law of Conservation of Mass: in a chemical reaction matter is neither created or destroyed. The total mass of sodium hydrogen carbonate will be equal to the total mass of water, carbon dioxide and sodium carbonate produced.
mass A = mass B + mass C + mass D
3360kG NaHCO3 = 360 kG H2O + 880 kG CO2 + mass NaCO3
Mass NaCO3 = 3360 kG - ( 360 kG + 880 kg)
The mass of the NaCO3 produced would be 2120 kG