(a)
Cell reaction is $Mg + {Sn}^{2 +} \rightarrow {Mg}^{2 +} + Sn$
$E_{cell} = E_{cell}^{{^\circ}} - \frac{0.0591}{2}\log\frac{\left\lbrack {Mg}^{2 +} \right\rbrack}{\left\lbrack Sn^{2 +} \right\rbrack}$
$= (2.34 - 0.14) - \frac{0.0591}{2}\log\frac{10^{- 2}}{10^{- 1}} = 2.23\ V$