\[\boxed{\text{138\ (138).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
Пояснение.
Решение.
\[\textbf{а)}\ \frac{m^{2} - 3m}{8x^{2}}\ :\frac{3m}{8x} =\]
\[= \frac{m(m - 3)}{8x²} \cdot \frac{8x}{3m} = \frac{m - 3}{3x}\]
\[\textbf{б)}\ \frac{5a^{2}}{6b^{3}}\ :\frac{a^{3}}{ab - b^{2}} =\]
\[= \frac{5a^{2}}{6b^{3}} \cdot \frac{b(a - b)}{a^{3}} = \frac{5(a - b)}{6ab²}\]