\[\left( - 5a^{3}b^{7} \right)^{3} \cdot \left( - \frac{1}{5}a^{2}c^{6} \right)^{2} =\]
\[= - 125a^{9}b^{21} \cdot \frac{1}{25}a^{4}c^{12} =\]
\[= - 5a^{13}b^{21}c^{12}\]