(1)(a-2b)(3a+4b)+(2a-4b)(2a-3b)=(a-2b)(3a+4b)+2(a-b)(2a-3b)
=(a-2b)[3a+4b+2(2a-3b)]
=(a-2b)(7a-2b)
(2)9(x-y)²+12(x²-y²)+4(x+y)²=9x²-18xy+9y²+12x²-12y²+4x²+8xy+4y²
=25x²-10xy+y²
=(5x-y)²