Van Der Waals Equation

Which of the following is/are correct about critical conditions of real gas following Vander wall's equation of state :

Consider the given plots for four gases at $300 \mathrm{K}$. If the slope of line $\mathrm{B}$ is $-0.10{\mathrm{Lmol}}^{-1}$, what is the relationship between Vander Waal's constants '$\mathrm{a}$' and '$\mathrm{b}$'? (Ignore the higher terms of Virial equation)

The critical constant for water are $401 \mathrm{K},218 \mathrm{atm}$ and $0.0566$ litre ${\mathrm{mol}}^{-1}.$ Calculate $\mathrm{a}\& \mathrm{b}$.