The coefficient of apparent expansion of mercury in a glass vessel is 153 × ${10}^{-6}$/°C and in a steel vessel is 144 × ${10}^{-6}$/°C. If $\alpha$ for steel is 12 × ${10}^{-6}$/°C, then that of glass is 

${\gamma }_{real}={\gamma }_{app}+{\gamma }_{vessel}$

. $153\times {10}^{-6}+{\left({\gamma }_{vessel}\right)}_{galss}=144\times {10}^{-6}+{\left({\gamma }_{vessel}\right)}_{steel}$. 

Further

 ${\left({\gamma }_{vessel}\right)}_{steel}=3\alpha =3\times \left(12\times {10}^{-6}\right)=36\times {10}^{-6}/°C$

$\Rightarrow 153\times {10}^{-6}+{\left({\gamma }_{vessel}\right)}_{glass}=144\times {10}^{-6}+36\times {10}^{-6}$

$\Rightarrow {\gamma }_{glass}=27\times {10}^{-6}/°C$

$\Rightarrow 3{\alpha }_{glass}=27\times {10}^{-6}/°C$

$\Rightarrow {\alpha }_{glass}=\text{ 9}\times {10}^{-6}/°C$