Step-by-step explanation:
[tex]a) \: - \frac{3}{7} a \: \: \times \: - \frac{21}{15} b[/tex]
[tex] = \frac{ 3}{5} ab \: (ans)[/tex]
[tex]b) \: 3xy(5 {x}^{2} {y}^{2} - 4yz)[/tex]
[tex] = 3xy \times 5 {x}^{2} {y}^{2} - 4yz \times 3xy[/tex]
[tex] = 15 {x}^{3} {y}^{3} - 12x {y}^{2} z \: (ans)[/tex]
