Your key to thermodynamic calculations.
Learn all your keywords, like in this case isothermal and reversible; and what they mean to a thermodynamic systems. In many calculations, just by knowing the keywords you can immediately recognize the answer is 0, which helps you greatly in calculating the other parts of the questions (but that is not the case here, just fyi).
In this case, the work is isothermal and reversible, the equation you are using is for constant pressure.
You must use your equation (or derive one, it is quite easy) for the isothermal expansion work.