Verified answer

Объяснение:

б)

[tex]21 {x}^{3} y \div \frac{7 {x}^{3} {y}^{2} }{3} = \frac{21 {x }^{3}y \times 3 }{7 {x}^{3} {y}^{2} } = \frac{3}{y} [/tex]

в)

[tex] \frac{m - 3n}{m + n} \times \frac{ {m}^{2} - {n}^{2} }{3m - 9n} = \frac{(m - 3n)(m - n)(m + n)}{3(m + n)(m - 3n)} = \frac{m - n}{3} [/tex]