Start by writing the atoms balance:
[tex] N_{2} -\ \textgreater \ 2N^{3-} [/tex]
Now, determine the change of oxidation states:[tex] N_{2} [/tex] has oxidation state 0, so each N has to gain 3 electrons to become [tex] N^{3-} [/tex].
That, means that you need 6 electrons to balance the charges, resulting in:
[tex] N_{2} + 6 e^{-} -\ \textgreater \ 2 N_{3-} [/tex]
And the answer is 6 mole of electrons.
