In this equation, what should be the coefficients of the reactants and products? I2 + O2 → I4O9

I am thinking that you are asking for the balanced equation for this chemical reaction. In order to balance this, you have to count each element where the elements in the reactants side and the product side should have equal number of molecules. The balanced reaction is as follows:

2 I2 + 9/2O2 → I4O9