\[\frac{\left( {- x}^{2} \right)^{5} \cdot x^{8}}{x^{21}} = \frac{- x^{10} \cdot x^{8}}{x^{21}} =\]
\[= - \frac{x^{18}}{x^{21}} = - \frac{1}{x^{3}};\]
\[x = \frac{1}{3}:\ \]
\[- 1\ :\left( \frac{1}{3} \right)^{3} = - 1\ :\frac{1}{9} = - 1 \cdot 9 = - 9.\]