(a+1)^2(a-1)^2-(a+2)^2(a-2)^2
=[(a+1)(a-1)]^2-[(a+2)(a-2)]^2
=(a^2-1)^2-(a^2-4)^2
=[a^2-1-(a^2-4)](a^2-1+a^2-4)
=3(2a^2-5)
=6a^2-15