The given inequality is expressed as
- 6 < 3n + 9 < 21
We would split the inequality into 2 parts
- 6 < 3n + 9
3n + 9 < 21
Solving the first part,
- 6 < 3n + 9
Subtract 9 from both sides of the inequality
- 6 - 9 < 3n + 9 - 9
- 15 < 3n
3n > - 15
n > - 15/3
n > - 5
Solving the second part, it beomes
3n + 9 - 9 < 21 - 9
3n < 21 - 9
3n < 12
n < 12/3
n < 4
Therefore, the solution is a combination of both solutions. It becomes
- 5 < n < 4