To find the heat of the reaction, you have to subtract the sum of the standard enthalpy of formation for the products by the sum of the standard enthalpy of formation of the reactants. Because both products are in their elemental forms, they are both zero. The heat of formation for ammonia is -46.3 kJ/mol.

0-2(-46.3 kJ/mol)=

0-(-92.6)= +92.6 kJ/mol.

You essentially had the correct answer numerically, however your sign should be positive instead of negative. Just think about it this way: You're decomposing ammonia into its constituent parts (hydrogen gas and nitrogen gas). If you're decomposing something, are you adding energy to the system in order to break apart the molecule, or is the system releasing energy? If energy is being released, it usually means something is being formed, not torn apart.