What is the dimensional formula of \( a b^{-1} \) in the equation \( \left( P + \dfrac{a}{V^{2}} \right) ( V - b ) = R T \), where letters have their usual meaning.
\( [ M^{4} L^{7} T^{4} ] \)
\( [ M^{0} L^{3} T^{-2} ] \)
\( [ M^{-1} L^{5} T^{3} ] \)
\( [ M L^{2} T^{-2} ] \)