Using Van der Waals's equation . Calculate the constant 'a' when two mole of a gas confined in a four litre flask exerts a pressure of 11.0 atmospheres at a temperature of 300K . The value of b is 0.05 lit $mol^{-1}$
$\begin{array}{1,1}(a)\;646\; atm\;litre^2\;mol^{-2}\\(b)\;6.46\;atm\;litre^2\;mol^{-2}\\(c)\;64.6 \;atm\;litre^2\;mol^{-2}\\(d)\;0.64\;atm\;litre^2\;mol^{-2}\end {array}$