\[81;\ - 54;36;\ldots \Longrightarrow q = - \frac{2}{3};\ \ \ \]
\[b_{n} = b_{1} \cdot q^{n - 1}\]
\[b_{6} = 81 \cdot \left( - \frac{2}{3} \right)^{5} =\]
\[= 81 \cdot \left( - \frac{32}{243} \right) = - \frac{32}{3} = - 10\frac{2}{3}\]
\[Ответ:\ b_{6} = - 10\frac{2}{3}.\]