The oxidation number of O is usually -2 and in this case it is.
Our ion is ReO₄⁻. It has a total charge of -1. The charge of the four atoms of O is -8. With this information we can write an equation. X will represent the oxidation state of Re.
Total charge = Oxidation state of Re + 4 * Oxidation state of O
- 1 = x + 4 * (-2)
- 1 = x - 8
-1 + 8 = x
x = +7
So the oxidation state of Re is +7.
