Q$2.$ a$)$ Prove that: $\frac{P_c{V}_c}{T_c}=\frac{3}{8}R?$

b$)$ van der Waals' constants for hydrogen chloride gas are $a=3\mathrm{.}67$atm$*L^{-2}$ and $b=40\mathrm{.}8mlmo{l}^{-1}.$ Find the critical temperature and critical pressure of the gas?