To simplify this expression, we can first simplify the denominator by using the exponent rule that states: (a^m)^n = a^(m*n).

So, we have:

2^3 * 6^5 = 2^3 * (2*3)^5 = 2^3 * 2^5 * 3^5 = 2^(3+5) * 3^5 = 2^8 * 3^5.

Now, we can rewrite the expression as:

12^5 / (2^8 * 3^5).

Next, we can use the exponent rule that states: a^m / a^n = a^(m-n).

So, we have:

12^5 / (2^8 * 3^5) = 12^(5-8) / 3^5 = 12^-3 / 3^5.

Finally, we can simplify further by using the exponent rule that states: a^-n = 1 / a^n.

So, we have:

12^-3 / 3^5 = 1 / (12^3 * 3^5).

Therefore, the simplified expression is 1 / (12^3 * 3^5).