∆Grxn = sum of ∆Gf (products) - sum of ∆Gf(reactants)
The reaction is
4NH3(g)+ 5O2 ---> 4NO(g)+6H2O(g)
∆Grxn = [ 4 X ∆Gf(NO) + 6 X ∆Gf(H2O)] - [ 4∆Gf(NH3) + 5 ∆Gf(O2)]
∆Grxn = [4 X (86.7) + 6 ∆Gf(H2O)] -[ 4 (-16.66) + 5(0)]
∆Grxn = [346.8 + 6∆Gf(H2O)] - [-16.66 + 0]
∆Grxn = -957.9 = [346.8 + 6∆Gf(H2O)] - [-66.64 + 0]
-957.9 = [346.8 + 6∆Gf(H2O)] - [-66.64 + 0]
∆Gf(H2O) = -90.74
This is based on information provided by you.
However the answer should be nearly -241.8