In H2S2O7, what are the oxidation numbers of oxygen?

The solution is as follows:


In H₂S₂O₇ oxidation number of:

            H = +1
             S = +6
            O = x

        2(+1) + 2 (+6) + 7(x) = 0
          2 + 12 + 7x  = 0
                14 + 7x = 0
                    7x = -14
                       x = -2