$a)V=P_p\cdot H=120\text{cm}\cdot 25\text{cm}=3000{\text{cm}}^3$  

$b)P_p={\left(9\text{cm}\right)}^2=81{\text{cm}}^2$  

$V=81{\text{cm}}^2\cdot 11\text{cm}=891{\text{cm}}^3$  

$c)P_p=\frac{a^2\sqrt{3}}{4}=\frac{{\left(10\text{cm}\right)}^2\sqrt{3}}{4}=\frac{100\sqrt{3}{\text{cm}}^2}{4}=25\sqrt{3}{\text{cm}}^2$  

$V=25\sqrt{3}{\text{cm}}^2\cdot 22\text{cm}=550\sqrt{3}{\text{cm}}^2$