We have
[tex]-6 < 3n+9 < 21 [/tex].
From this we have system of inequality
[tex]
-6 < 3n+9 \\
3n+9 < 21[/tex]
We have to simplify both inequalities
[tex]
3n+9 > -6 \Longrightarrow n > -5 \\
3n+9 < 21 \Longrightarrow n < 4
[/tex]
So we can conclude that
[tex]n > -5 \wedge n < 4 \Longrightarrow x\in(-5,4)[/tex]
Hope this helps.
