a)
2^{18} * 8^{-4} : 16 = 2^{18} * (2^3)^{-4} : 2^{4} = 2^{18} * 2^{-12} : 2^4 = 2^6 : 2^4 = 2^2 = 4
b)
27^{-4} : 81^{-3} * 9^{12} = 2^3 * 3^{18} = (3^3)^{-4} : (3^4)^{-3} * (3^2)^{12} = 3^{-12} : 3^{-12} * 3^{24} =
= 3^{-12+12} * 3^{24} = 3^{0+24} = 3^{24}