(3a-1/2b)^2(9a^2+1/4b^2)^2(3a+1/2b)^2
=[(3a-1/2b)(3a+1/2b)(9a^2+1/4b^2)]^2
=[(9a^2-1/4b^2)(9a^2+1/4b^2)]^2
=(81a^4-1/16b^4)^2