$C{u^{ + + }} + {e^ - } \to C{u^ + };\,\,\,E_{C{u^{ + + }}/C{u^ + }}^o = 0.15\,\,V\,\,$ …..$(i)$
$C{u^{ + + }} + 2{e^ - } \to Cu;\,\,\,\,E_{C{u^{ + + }}/Cu}^o = 0.\,34\,\,V\,$ …..$(ii)$
Multiplying eq. $(i)$ by $2$ we get
$2C{u^{ + + }} + 2{e^ - } \to 2C{u^ + }$ …..$(iii)$
$\Delta {G_1} = - nFE = - 2 \times F \times 0.15$
$C{u^{ + + }} + 2{e^ - } \to Cu\,\,\,$…..$(iv)$
$\Delta {G_2} = - nFE = - 2 \times F \times 0.34$
Subtract the eq. $(iv)$ from $ (iii)$
$C{u^{ + + }} + Cu \to 2C{u^ + }$
$\Delta {G_3} = - \,nFE = - 1 \times F \times {E^o}$
Also $\Delta {G_3} = \Delta {G_1} - \Delta {G_2}$
$ - 1F{E^o} = ( - \,2F \times 0.15) - ( - \,2F \times 0.34)$ ${E^o} = - \,0.38$
This is the value for the reaction
$C{u^{ + + }} + Cu \to 2C{u^ + }$
But the given reaction is just reverse of it
${E_{{\rm{cell}}}}$ for given reaction $= + 0.38\,V.$
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