$$K=3a^{2}b\times(4ab^{2})^{2}$$
$K=48a^{4}b^{5}$
$\left\{\begin{matrix}a=\frac{3^{\frac{3}{4}}\sqrt[4]{\frac{K}{b^{5}}}}{6}\text{; }a=-\frac{3^{\frac{3}{4}}\sqrt[4]{\frac{K}{b^{5}}}}{6}\text{, }&\left(K\leq 0\text{ and }b<0\right)\text{ or }\left(K\geq 0\text{ and }b>0\right)\\a\in \mathrm{R}\text{, }&K=0\text{ and }b=0\end{matrix}\right.$