H2 does not have a coefficient.
When writing chemical equations, coefficient refers to the numerical values written at the back of each compound, which show the number of molecule of each substance that is involved in the reaction. For instance, in 2H20, 2 is the cofficient, which tell us that two molecules of water is involved.
In the question given, the figure 2 attached to the symbol H is a subscript, which shows the number of atoms in the hydrogen molecule.
