we have
[tex](5x^{7} y^{2})(-4 x^{4} y^{5})=-20 x^{a} y^{b}[/tex]
Multiply the terms of the left side of the equation and then compare with the right side to find the values of a and b
[tex](5x^{7} y^{2})(-4 x^{4} y^{5})=-20 x^{(7+4)} y^{(2+5)}[/tex]
[tex]=-20 x^{11} y^{7}[/tex]
substitute
[tex]-20 x^{11} y^{7}=-20 x^{a} y^{b}[/tex]
Solve for a
[tex]x^{11} =x^{a}[/tex]
[tex]a=11[/tex]
Solve for b
[tex]y^{7} =y^{b}[/tex]
[tex]b=7[/tex]
therefore
the answer is
[tex]a=11[/tex]
[tex]b=7[/tex]
