Molecular mass of a compound is equal to the sum of the masses of its constituent atoms. The atomic mass of sodium, carbon and oxygen  are 23u, 12u and 16u respectively.

Therefore, the molecular mass of ${Na}_2{CO}_3$
$=(2\times \text{atomic mass of Na})$ + $(1\times \text{atomic mass of C})$ + $(3\times \text{atomic mass of O)}$
$=(2\times 23)+(1\times 12)+(3\times 16)$ 
$=46+12+48=106u$.