We are going to use these properties :
1. (a^m)^n = a^mn
2. a^m x a^n = a^(m+n)
3. a^m : a^n = a^(m-n)
Using the first property, (3^6)^5 = 3^(6x5) = 3^30
Using the second property :
9 = 3^2
-> 9 x 3^7 = 3^2 x 3^7 = 3^(2+7) = 3^9
Therefore, it is simplified down to 3^30 : 3^9
Finally, we will use the third property :
3^30 : 3^9 = 3^(30-9) = 3^21