A-{B+[C-(A-B)]}
=A-{B+[C-A+B]}
=A-{B+C-A+B}
=A-2B-C+A
=2A-2B-C
=2(A-B)-C
=2(2x²-3x+2y²-2x²-xy+3y²)-(x²-xy-2y²)
=-6x+10y²-2xy-x²+xy+2y²
=12y²-xy-x²-6x