By principle of dimensional homogenity $\left\lbrack \frac{a}{V^{2}} \right\rbrack = \lbrack P\rbrack$ $\therefore\ \lbrack a\rbrack = \lbrack P\rbrack\lbrack V^{2}\rbrack = \lbrack ML^{- 1}T^{- 2}\rbrack \times \left\lbrack L^{6} \right\rbrack = \lbrack ML^{5}T^{- 2}\rbrack$