Carbon dioxide gas means CO₂(g)
solid barium oxide means BaO(s)
solid barium carbonate means BaCO₃(s)
The reaction is:
CO₂(g) + BaO(s) --> BaCO₃(s)
We now need to balance it.
We have 1 carbon atom on the left side and 1 carbon atom on the right side
We have 3 oxygen atoms on the left side and 3 oxygen atoms on the right side
We have 1 barium atom on the left side and 1 barium atom on the right side
Therefore, the chemical equation for the reaction is balanced.
CO₂(g) + BaO(s) --> BaCO₃(s)