(16a^2)^3*1/8^4=9
(4a)^6=9*8^4
(4a)^6=9*4^4*2^4
4^6*a^6=9*4^4*4^2
4^6*a^6=9*4^6
a^6=9
a^12=(a^6)^2=9^2=81