How do you find the standard reduction potential for a half reaction when you are given two other half reactions of the same element but with different charges? The question expects you to use the "extensive property of ?G).

M^3+ + 3e- ---> M     E0 = -0.10V

M^2+ + 2e- ---> M      E0 = -0.50V

Using those two standard reduction potentials, solve for the standard reduction potential of the half reaction

M^3+ + e- ---> M^2+