Bana cevabın vermedin - does that mean: "You didn't give me your answer?"
I think then you will need accusative ending -ı to cevabın.
What is the difference between vermedin and vermedi
Vermedin - you did not give.
Vermedi - s/he did not give.
What is the infinitif of vermedin?
Vermek = to give ; vermemek = not to give.