How would you do that?

Hint#1: there are some formulas that link a and b to the critical values. You can get them starting from the definition of critical point and using van der Waals equation. Think at the isotherms of a real gas.

Hint#2:

$$

\left\{ \begin{array}{l}

\left({\frac{{\partial p}}{{\partial V}}} \right)_T=0\\

\left({\frac{{\partial ^2 p}}{{\partial V^2 }}} \right)_T=0\\

\end{array} \right.

/$$

I don't know if there's an easier way to find them out but I'll think at it