（x+2y+3）²-(x-2y+3)(x-2y-3)
=x^2+4y^2+9+4xy+6x+12y-(x-2y)^2+3^2
=x^2+4y^2+9+4xy+6x+12y-x^2-4y^2+4xy+9
=18+8xy+6x+12y