Only Fe2+ reacted with the oxidising agent to form Fe3+. Fe3+ remains at it's original oxidation state. Write out the balanced eqn for the redox equation using the electrolysis half equations given in your data booklet. Take note the eqn takes place in an acidic medium. You can then find the amt of fe2+ reacted and hence it's mass in the solution. Hope this helps!

I mentioned this in one of your previous questions. If a reaction is involved always write out the balanced equation if you are stuck. Applies more so to questions involving more than one reaction. It simplifies the question and your calculations would have lesser careless mistakes. :)