Fe (NO3) 2 not sure how to get the oxidation numbers of all elements

Fe (NO3) 2

The NO3 part is a poly-atomic ion with total charge - 1.

This is because Fe has a + 2 charge and two NO3's with a - 1 charge will balance out to 0.

Most often we just make the assumption that Oxygen has a - 2 oxidation number because it is very electro-negative.

So to find N, we just need an oxidation number that balances out with 3 (-2) to get - 1 (the total charge of the ion)