x-y=3
（x-y)^2=9
x^2-2xy+y^2=9
x^2+y^2=13
（x+y）（x-y）-y^2-（6x^2y-2xy^2)/2y
=x^2-y^2-y^2-3x^2+xy
=-2(x^2+y^2)+xy
=-2*13+2
=-24