The oxidation numbers of nitrogen in NH4+, NO2, and NaNO3 are, respectively:

-3, + 4, + 5

+3, + 5, + 4

+3, + 5, - 4

-3, - 4, + 5

Hydrogen has a + 1charge and the whole thing has a + 1charge so

(N) + 4 (H) = + 1

and plug in H you get

(N) + 4 (1) = 1 which means N has a charge of - 3

If you use that set up for the rest you should be able to figure them out if there is no overall charge you just set it equal to 0