Mn(2+), Cr(6+), O(-2)
Explanation:
The typical oxidation state of oxygen is -2. Manganese typically takes a +2 oxidation state. This leaves chromium to balance the oxidation state(the oxidation states must sum to zero in a neutral compound).
Thus, 2 – 8 + Cr = 0
We have Cr(6+) which is its maximum oxidation number.