a)
\frac{(x^5)^4*(x^2)^3}{(x^3)^5}=\frac{x^{20+6}}{x^{15}}=x^{26-15}=x^{11}
b)
\frac{9a^6b^5}{10c^4d^3}:\frac{12a^4b^7}{5c^2d^5}=\frac{\not9^3a^6b^5}{\not10^2c^4d^3}*\frac{\not5^1c^2d^5}{\not12^4a^4b^7}=\frac{3a^6b^5c^2d^5}{8a^4b^7c^4d^3}=\frac{3}{8}*a^{6-4}b^{5-7}c^{2-4}d^{5-3}=
=\frac{3}{8}*a^2*b^{-2}*c^{-2}*d^2=\frac{3a^2d^2}{8b^2c^2}