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1. compounds must have zero charge.
2. ions must have charge on the ion.
3. oxidation states in the compound/ion must add to the charge.
For SeO4^2-, youhave
O is -2 and 4*-2 = -8 so Se must be +6 t leave a charge of -2 on the ion.
For H2S, H is +1, 2*+1 = +2 so S must be -2. The right hand side is done the same way.
The oxidizing agent (oxidant) is the one that gained electrons.