Not all the equations show up... =/

But I suppose that's what are given and this is how I work them out:

You are already given which is -91.8\ kJ/mol. (and this I think is the enthalpy ofreactionand notformationand hence, you do not divide by 2)

Then, (since you have 3 mol of water being formed from my Hess Cycle)

Then, (I think they gave you the standard enthalpy of combustion of ammonia. So, for each mol of ammonia burnt, you get 906.2 kJ but since you burn 2 moles, it's twice)

Then, This is what you need to find I think.

Then, from Hess' Law, we find that by following the arrows:

This will give you the answer.