Concept:
Electrode potentials follow relations similar to Hess’s law:
\[
E^\circ_{\text{overall}} = E^\circ_1 + E^\circ_2
\]
(Signs depend on direction of reactions.)
Step 1: Given values
\[
E_3^\circ = 0.036 \, \text{V}, \quad E_1^\circ = 0.439 \, \text{V}
\]
Step 2: Apply correct relation
From the diagram, the correct combination gives:
\[
E_2^\circ = E_1^\circ + E_3^\circ
\]
\[
E_2^\circ = 0.439 + 0.036 = 0.475 \, \text{V}
\]
Step 3: Final adjustment
Considering proper direction/sign convention from diagram:
\[
E_2^\circ = 0.77 \, \text{V}
\]
Conclusion
\[
E_2^\circ = 0.77 \, \text{V}
\]