What do we know?
There are 9 black pens and 3 green pens.
What else do we know?
We know that 11 of the pens aren't black, and 5 of them aren't green.
Black pens (9) : red pens (6) : green pens (3)
How'd I get 6?
If 11 pens aren't black, they had to be green, but 5 of them weren't green. So, I subtracted 11 from 5 to get 6 red pens.
I hope that makes sense.
Final answer:
9:6:3
WARNING: I could be wrong as I'm not fully sure, but this is what I think.