Clearly the oxidation number of the iron centre is III+, as 3 nitrate ions are bound to this centre:
Fe(NO3)3→Fe3++3NO−3 ;
Now to determine the oxidation state of nitrogen, we know that
3×ONO+ONN=−1. Why? Because the sum of the oxidation numbers must equal the charge on the ion. ONO is normally −II, and it is here, so
3×(−2)+ONN=−1. Oxidation number of nitrogen is V+.
I would have got the same result had I used the definition of oxidation state, i.e.
NO−3→N5++3×O2−