7, 21, 8, 72, 9, 

Start with 7

First we have 7.
7 times 31 gives 21
 Next we have 8.
8 times 32 gives 72
 Next we have 9.
9 times 33 gives 243
 Next we have 10.
10 times 34 gives 810
 Next we have 11.
11 times 35 gives 2673
 Next we have 12.
12 times 36 gives 8748
...

Edwin