\[\left( - x^{6} \right)^{7} \cdot \left( - x^{3} \right)^{3}\ :x^{15} =\]
\[= - x^{42} \cdot \left( - x^{9} \right)\ :x^{15} =\]
\[= x^{42 + 9 - 15} = x^{36}\]