$\left( P +\frac{ a }{ V ^{2}}\right)( V - b )= RT$

The dimension of a is given as,

$a = PV ^{2}$

$=\left[ ML ^{-1} T ^{-2}\right]\left[ L ^{3}\right]^{2}$

$=\left[ ML ^{5} T ^{-2}\right]$

The dimension of b is given as,

$b = V$

$=\left[ L ^{3}\right]$

Thus, the dimension of a is $\left[ ML ^{5} T ^{-2}\right]$ and the dimension of $b$ is $\left[ L ^{3}\right]$.