Question

why decomposition reaction called opposite of combination reaction write equation for this reactions

Answer 1

because in decomposition reaction the biggest molecule dissociates to form smaller molecule whereas in combination reaction the smaller molecule joined together to form bigger molecule
2H2+O2gives 2H2O

Answer 2

Decomposition reaction is called opposite of combination reaction because in decomposition reactions a single substance decomposes to form two or more substances and in combination reaction two or more reactants combine to form a single product.
ex.: Decomposition reaction: CaCO3 (s) ⇨ CaO (s) + CO2 (g) 

 Combination reaction: H2 (g) + Cl2 ⇨ 2HCl (g)