Answer: The coefficient '2' must be added in front of
to form a balanced equation.
Step-by-step explanation:
Every balanced chemical equation follows law of conservation of mass.
This law states that mass can neither be created nor be destroyed but it can only be transformed from one form to another form. The total number of individual atoms on the reactant side must be equal to the total number of individual atoms on the product side.
For the given chemical equation:

The coefficient '2' must be added in front of
to balance the equation.