Question

What is the formula for the compound iron (iii) sulfate?

Answer 1

The Iron (III) ion is  $Fe^{3+}$
The sulfate ion is $SO _{4}^{2-}$

Thus when forming the compound iron (iii) sulfate you would need three sulfate ions bonded to two iron ions in order to fill the valency requirement 

∴ the formula would be   Fe₂(SO₄)₃

Whenever you are adding a number to a radical you must bracket it.

Answer 2

If your're on plato it's Fe₂(SO₄)₃