(a+2b)²-4(a-b)²
=[(a+2b)+2(a-b)][(a+2b)-2(a-b)]
=(3a)(-a+4b)
=3a(4b-a)
18a³-12a²b+2ab²
=2a(9a²-6ab+b²)
=2a(3a-b)²