Both reactions (combination and decomposition) are similar but opposite to each other. 

The combination reaction occurs when two reactants react to form a new product. The generalised form of the reaction can be understood by 

X + Y → XY

Example: Ba (s) + F₂ (g)→BaF₂ (s)

The decomposition reaction occurs when the compound breaks into two or more simpler compounds.

XY → X+Y

Example: 2HgO(s)→2Hg(l)+O₂(g)

As seen from the reactions above, you can see how they are opposite of each other.