Ответ: -140x⁸y⁴

[tex]20 {x}^{5} {y}^{3} \times ( - 7 {x}^{3} y) = - 140 {x}^{8} {y}^{4} [/tex]

[tex]\displaystyle\bf\\20x^{5} y^{3}\cdot(-7x^{3} y)=\underbrace{20\cdot(-7)}_{-140}\cdot x^{5+3} \cdot y^{3+1} =-140x^{8} y^{4}[/tex]