Mn O.N =+7
We all know oxygen has an oxidation number = 2
So when we have a charge on the molecule.
It means the sum of all the oxidation number of the molecule should add up to give that charge.
Like the ex. You have asked
The molecule has -1 as the charge.
(O.N of Mn + O.N of Oxygen should equal to -1)
We all know oxidation number(O.N) of O is -2
Let X be the O.N of Men
Hope you understand. :)