=6x^2-15xy+2xy-5y^2-6x^2+12xy-6y^2=-11y^2-xy
(3x+y)(2x-5y)-6(x-y)^2=6x^2-13xy-5y^2-6x^2+12xy-6y^2=-13xy-5y^2+12xy-6y^2=-xy-5y^2-6y^2=-xy-11y^2
=6x^2-15xy+2xy-5y^2-6x^2+12xy-6y^2=-11y^2-xy
(3x+y)(2x-5y)-6(x-y)^2=6x^2-13xy-5y^2-6x^2+12xy-6y^2=-13xy-5y^2+12xy-6y^2=-xy-5y^2-6y^2=-xy-11y^2