(3a+2b)^2+(3a-2b)^2-(6a-4b)(3a-2b)
=(3a+2b)^2+(3a-2b)^2-2*(3a-2b)(3a-2b)
=(3a+2b)^2-(3a-2b)^2
=(3a+2b+3a-2b)[3a+2b-(3a-2b)]
=6a*4b=24ab