Verified answer

Well, in order for it to be a combustion reaction, NaHCO3 must react with oxygen and produce carbon dioxide and water.

But it isn't a decomposition reaction either because NaHCO3 doesn't break up.

It looks more like a single replacement, and then a decomposition of H2CO3, in other words..

NaHCO3+HCI(aq) ⇢ NaCl(aq) + H2CO3(aq) ⇢ CO2(g) +NaCl(aq) + H2O(l)

Hope this help (:

Decomposition