Let x be the oxidation number of Mn in MnO2.
Since the overall charge on the complex is 0, the sum of oxidation states of all elements in it should be equal to 0.
This compound, called manganese dioxide, also contains two oxygen ions, each with an oxidation number of -2. The manganese ion also exhibits other oxidation numbers, such as +2 and +7. For example, in KMnO4, the potassium ion has a charge of +1 and each of the oxygen ions has a charge of -2, which causes the manganese ion to take a charge of +7.