So there's the compound.

The oxidation state of each oxygen is -2.

That's the first rule that we come across if we look at the oxidation state rules.

There is 4 oxygens, so -2 times 4

means that those 4 oxygens equal -8.

Now we have memorized that the charge on sulfate is 2-, so inside this parentheses,

everything inside that parentheses needs to add up to 2- or -2.

And you'll notice sometimes that I put the charge after the number and

sometimes I put the charge before the number.

Those are both acceptable conventions, so just do it any way you want.

Don't worry about it.

In order for if this oxygen is -8 for the 4 oxygen, sulfur needs to be what then?

Sulfur needs to be +6.

So here sulfur is +6.

So everything now inside the parentheses is -2.

And there's 2 of those, so

2 times -2 equals -4, right?

So all of this together is -4.

So the charge on iron then must be +4.