Find a context-free grammar for the language $L=\{a^nb^m\mid 2n<m<3n\}$
Hint: Can you do $$L=\{a^nb^m\mid m=3n\}$$

Try it also for: $$L=\{a^nb^m\mid m=3n-1\}$$

Then you might want to be able not to always have that many $b$.

And there is a bit more to take care of.