Answer:
If the reaction is going from left to right it would be synthesis, if going right to left it would be decomposition.
Step-by-step explanation:
Na2O (s) + 2CO2 (g) + H2O (g) = 2 NaHCO3 (s)
Left Side
Na = 2
O = 6
C = 2
H = 2
Right Side
Na = 2
O = 6
C = 2
H = 2
Ok so is balanced already, though initial equation in question had an error in regards to the left hand side the ) should have been an O.
Hmm
Na2O (s) + 2CO2 (g) + H2O (g) = 2 NaHCO3 (s)
If it was going from right to left would be decomposition
But going from left to right it would be synthesis