(x-2y+3)(x+2y-3)-(x-2y-3)(x-2y-3)
=[(x-2y-3)+6][(x-2y-3)+4y]-(x-2y-3)(x-2y-3)
=6(x-2y-3)+4y(x-2y-3)+24y
=6x-12y-18+4xy-8y^2-12y+24y
=6x-18+4xy-8y^2