generally  SO ₄    

ion is in the form   SO₄^-2     .

we know that the oxidation no of oxygen is -2  & the sum of oxidation no of the elements in a compound is equal  to the charges on the compound. In SO₄^-2                            

let the ox. no . of s be x & that of O  is -2  hence x + 4(-2)=-2

x-8=-2

x=6