\[6;\ \ x;\ \ y;\ - 18\sqrt{3} \Longrightarrow\]
\[\Longrightarrow геометрическая\ \]
\[прогрессия \Longrightarrow\]
\[\Longrightarrow q = \frac{x}{6} = \frac{y}{x} = - \frac{18\sqrt{3}}{y}\]
\[\left\{ \begin{matrix} \frac{x}{6} = \frac{y}{x}\text{\ \ \ \ \ \ \ \ \ \ } \\ \frac{y}{x} = - \frac{18\sqrt{3}}{y} \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ \ \ }\]
\[\left\{ \begin{matrix} y = \frac{x^{2}}{6}\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ } \\ x^{3} = - 648\sqrt{3} \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ }\]
\[\left\{ \begin{matrix} y = \frac{x^{2}}{6}\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ } \\ x^{3} = \left( - \sqrt{108} \right)^{3} \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ }\]
\[\left\{ \begin{matrix} x = - \sqrt{108} \\ y = 18\ \ \ \ \ \ \ \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ \ \ \ }\]
\[\left\{ \begin{matrix} x = - 6\sqrt{3} \\ y = 18\ \ \ \ \ \\ \end{matrix} \right.\ \]
\[Ответ:\ - 6\sqrt{3}\ \ и\ \ \ 18.\]