You can see that NO₂⁻ has an oxidation number of -1 and V³⁺ has an oxidation number of +3.
When we state the chemical formula of these compounds, we need to have that the algebraic sum of the oxidation numbers must be zero. In this case, we have that the sum is -1 + 3 = 2, it is incorrect because we need to have the result of zero.
For this problem, we can multiply three times NO₂⁻ obtaining that the oxidation number is -3, so the algebraic sum would be: (-1)(3) + 3 = -3 + 3 = 0. This means that the chemical formula will have three times NO₂⁻ bonded with V³⁺ and it would be:
