(6ac^3)2 : (9a^2c^5)=

=(6^2 a^2 c^6) : (3^2 a^2 c^5)
=(3^2*2^2 a^2 c^6) : (3^2 a^2 c^5)
=3^(2-2) * 2^2 a^(2-2) c^(6-5)
=3^0 * 2^2 a^0 c
=1 * 2^2 * c