Решение:

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

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

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