Question

x=0,3 y=-10

Answer 1

$3 x^{2} -(7xy-4 x^{2} )+(5xy-7 x^{2} ) =3 x^{2} -7xy+4 x^{2} +5xy-7 x^{2} = \\ =3 x^{2} +4 x^{2}-7 x^{2} -7xy+5xy =-2xy \\ x=0.3 \\ x=-10 \\ -2xy=-2*0.3*(-10)=-2*(-3)=6$