Describe what subscripts and coefficients indicate in chemistry notation

The hint here is to look at the anion of the compound. That is the polyatomic ion, sulfate or SO4. It has a charge of -2. Since the compound does not show any subscript, then it means that the cation must have a charge of +2 for it to be cancelled out. Hence, the elements that can represent X are the elements that have a +2 charge like Fe, Ca, Ba, Mg, Zn, etc.