\((\frac{3}{5}a^{-8}b^{-7})^{-3} \cdot (-5a^6b^{12})^{-2} = (\frac{5}{3})^3 a^{24} b^{21} \cdot \frac{1}{(-5)^2 a^{12} b^{24}} = \frac{5^3}{3^3} a^{24} b^{21} \cdot \frac{1}{25 a^{12} b^{24}} = \frac{125}{27} \cdot \frac{1}{25} \cdot a^{24-12} b^{21-24} = \frac{5}{27} \cdot a^{12} b^{-3} = \frac{5a^{12}}{27b^3}\)