Решение:

a) $$rac{35a^4b^8}{21a^2b^4} = rac{5a^2b^4}{3}$$

б) $$rac{15x^2}{6x+15x^2} = rac{15x^2}{x(6+15x)} = rac{15x}{6+15x} = rac{5x}{2+5x}$$

Ответ: a) $$rac{5a^2b^4}{3}$$; б) $$rac{5x}{2+5x}$$