Given, $a = 3bc^{2}$ $\Longrightarrow \ b = \frac{a}{3c^{2}}$ Writing dimensions for $a$ and $c$, we have $\lbrack b\rbrack = \frac{\lbrack Q/V\rbrack}{\lbrack B\rbrack^{2}} = \frac{\lbrack Q\rbrack\lbrack ML^{2}T^{- 2}Q^{- 1}\rbrack}{\left\lbrack MT^{- 1}Q^{- 1} \right\rbrack^{2}}$ $= \lbrack M^{- 3}L^{- 2}T^{4}Q^{4}\rbrack$