7(a-b)²+8(a+b)²-9(a-b)²-4(a+b)²
=7(a-b)²-9(a-b)²+8(a+b)²-4(a+b)²
=-2(a-b)²+4(a+b)²
=-2a²-2ab+b²=4a²+2ab+b²
=2a²+12ab+2b²