2(2x-3y)的2次幂-8(3x-2y)的2次幂
=2(4x^2-12xy+9y^2)-8(9x^2-12xy+4y^2)
=(8x^2-24xy+18y^2)-(72x^2-96xy+32y^2)
=-64x^2+72xy-14y^2