In every chemical reaction, mass is always conserved. This implies that in chemical reaction, the process proceeds maintaining the same set of atom in the same proportion without new ones forming.
From the description given, decomposition of the Silver carbonate will produce a silver residue with the given mass.
The other mass that seems lost can be examined to be given off as carbon dioxide gas which is the other product of the reaction and oxygen.
Therefore, since the products are silver, carbon dioxide and oxygen, the remaining mass is that of the carbon dioxide and oxygen. It is not lost.
