（1）(-2ab)(3a^2-2ab-4b^2)
=-6a^3b+4a^2b^2+8ab^3
(2)(2a+b/3)(b/3-a/2)
=1/2(ab)-a^2+1/9(b^2)
(3)x+y-3
(4)(2x-3)^2(2x+3)^2
=(4x^2-9)^9
=16x^4+81-72x^2