If $E^0_{Fe^{+2}/Fe}$ is $X_1$,$E^0_{Fe^{+3}/Fe}$ is $X_2$ then $E^0_{Fe^{+3}/Fe^{+2}}$ will be

Question

$\begin{array}{1 1}(a)\;3X_2-2X_1\\(b)\;X_2-X_1\\(c)\;X_2+X_1\\(d)\;2X_1+3X_2\end{array}$