(d) $\gamma_{real} = \gamma_{app} + \gamma_{vessel};\gamma_{vessel} = 3\alpha$
For vessel $'A^{'} \Rightarrow \gamma_{real} = \gamma_{1} + 3\alpha$
For vessel $'B^{'} \Rightarrow \gamma_{real} = \gamma_{2} + 3\alpha_{B}$
$Hence,\ \gamma_{1} + 3\alpha = \gamma_{2} + 3\alpha_{B} \Rightarrow \alpha_{B} = \frac{\gamma_{1} - \gamma_{2}}{3} + \alpha$