Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions?

In a decomposition reaction, a single reactant breaks down to give simpler products.

CaCO₃ $\xrightarrow{\Delta}$ CaO + CO₂

In a combination reaction, a single product is formed from two or more reactants.

CaO + H₂O ⟶ Ca(OH)₂

Hence, we can say that the decomposition reaction is opposite of combination reaction.