\[\frac{6a - 15b}{a^{6}x}\ :\frac{(10a - 25b)}{a^{3}x^{5}} =\]
\[= \frac{(6a - 15b)a^{3}x^{5}}{a^{6}x(10a - 25b)} =\]
\[= \frac{3(2a - 5b)x^{4}}{a^{3} \cdot 5(2a - 5b)} = \frac{3x^{4}}{5a^{3}}.\]