\[b_{2} = 6;\ \ \ \ b_{4} = 18:\]
\[\left\{ \begin{matrix} b_{1}q = 6\ \ \ \ \\ b_{1}q^{3} = 18 \\ \end{matrix}\text{\ \ \ \ \ } \right.\ \]
\[\left\{ \begin{matrix} b_{1}q = 6\ \ \ \ \ \ \ \ \ \ \\ b_{1}q \cdot q^{2} = 18 \\ \end{matrix}\text{\ \ \ \ \ } \right.\ \]
\[6q^{2} = 18\ \]
\[q^{2} = 3\]
\[q = \pm \sqrt{3}.\]
\[Ответ:\ \ q = \pm \sqrt{3.}\]