a,\(\frac{3^{10}.\left(-5\right)^{21}}{\left(-5\right)^{20}.3^{12}}=\frac{3^{10}.\left(-5\right).\left(-5\right)^{20}}{\left(-5\right)^{20}.3^{10}.3^2}\)\(=\frac{-5}{3^2}\)
b,\(\frac{-11^5.13^7}{11^5.13^8}=\frac{-11^5.13^7}{\left(-11\right)^5.\left(-1\right)^5.13^7.13}\)\(=\frac{1}{-1^5.13}\)
\(\frac{3^{10}.\left(-5\right)^{21}}{\cdot\left(-5\right)^{20}.3^{12}}=\frac{\left(-5\right)}{3^2}=\frac{-5}{9}\)
\(\frac{\left(-11\right)^5.13^7}{11^5.13^8}=\frac{-1}{13}\)