The working equation for this is written below:
E° = 0.0592logK/n
So, we have to know the value of n first which represents the number of moles electron in the reaction. We cannot answer this because we are not given with the reaction. However, just suppose the reaction is:
Cu⁺ + 2e⁻ --> Cu
Then, n=2. Continuing,
E° = 0.0592log(6.47×10⁵)/2 = 0.172 V