Given at 298 K :
$$
\begin{aligned}
& \mathrm{E}_{\mathrm{Fe}^{2+} / \mathrm{Fe}}^{!}=\mathrm{X} \text { Volt } \\
& \mathrm{E}_{\mathrm{Fe}^{3+} / \mathrm{Fe}}^{!}=\mathrm{Y} \text { Volt }
\end{aligned}
$$
The $\mathrm{E}_{\mathrm{Fe}^{3+} / \mathrm{Fe}^{2+}}^{!}$in Volt at 298 K is given by :