7,21,8,72,9,
Start with 7:
multiply 7 by 31 or 3,
get 7*3=21.
Next term is 8.
multiply 8 by 32 or 9,
get 8*9=72.
Next term is 9.
multiply 9 by 33 or 27,
get 9*27=243.
Next term is 10.
multiply 10 by 34 or 81,
get 10*81=810.
Etc., etc.
7,21,8,72,9,243,10,810,11,2673,12,8748,13,28431,14,91854,15,295245,...
Edwin