Question

oxidation numbers ClO2-

Answer 1

You can see that the polyatomic ion has a charge of -1. So, the charges must add up to -1.
First, what is the 'usual' oxidation state of oxygen?
Once you have that, since you have two of them, multiply it by 2.
(I will tell you that that will be a negative number.)
Once you know that number, what value must you add to it to add up to a total charge of -1? That will be the oxidation number of the Cl.
So, based on this, what do you believe the oxidation state of (each) oxygen, and of the chorine is?

Answer 2

to simply answer your question its -3. :)

Answer 3

Are you sure about that be? If the oxidation number of the Cl was -3, the polyatomic ion would have a charge of -7, not -1.