а)

[tex] - 5 {m}^{4} {n}^{5} \times 4 {n}^{3} \times {m}^{7} = ( - 5) \times 4 {m}^{4 + 7} {n}^{5 + 3} = - 20 {m}^{11} {n}^{8} [/tex]

б)

[tex]( - 2 {m}^{3} n) ^{2} = {( - 2)}^{2} {m}^{3 \times 2 } {n}^{2} = 4 {m}^{6} {n}^{2} [/tex]