4xy-((x^2+5xy-y^2)-(x^2+3xy-2y^2))
=4xy-(x²+5xy-y²)+(x²+3xy-2y²)
=4xy-x²-5xy+y²+x²+3xy-2y²
=2xy-y²
当x=2,y=-1时
原式=-4-1=-5