The first thing that has to be determined in calculating theoretical mass is if the limiting reactant is known. In this particular case it is not because you had the choice in measuring the reactants involved. You do not really know which of the two reactant was in excess and which limits the amount of product made. Therefore, the best method used, which will cover most of your bases, is to just do a mass-to-mass problem for each of the reactant finding the product.
The outline of what needs to take place is as follows:
Grams of sodium carbonate ® moles of sodium carbonate ® moles of calcium carbonate ® grams of calcium carbonate
Grams of calcium nitrate ® moles of calcium nitrate ® moles of calcium carbonate ® grams of calcium carbonate
Notice that both end in calcium carbonate. This is because your seeing how much calcium carbonate is possible to make from each of the different reactants. The lower one will be the limiting reactant because that is all that can possibly be made from that reactant.
This is all calculated in theory or by calculation. Therefore they call this theoretical mass. The experimental mass is the mass that is actually made in the lab.