2.0 g of a mixture of and was heated when its weight reduced to 1.876 g. determine the percentafe composition of the mixture

For example in this ap chem lab i have to calculate the percent of Na2CO3 and the percent of NaHCO3 in this mixture. I have 4g of the mixture. Percent comp is total mass of element/ molar mass of the compound but how can i calculate that with the compound i have here? Also with the data of 4g original mixture and 3.19g after heating, how can i calculate the atomic mass of Na2CO3