(x+2y)^2-(x+y)(3x-y)-5y^2
=x^2+4xy+4y^2-3x^2-2xy-y^2-5y^2
=-2x^2+2xy-2y^2
=2(-x^2+xy-y^2)
x=-2,y=1/2则xy=-1,代入得-10.5