N₂H₄   ,X

here in  N₂H₄ the O.N. of H is -4 and that of N is +4

when X is formed N looses 10 electrons so the  net charge on  N₂ =+4-10= -6

since 1 molecule of N  has 2 atoms  so one atom has    -  6/2=- 3   ON

hence N has -2 ON