How to create a (N)PDA for L={w∈{a,b}⋆ |w=(a2b)n mitn∈N0}?

I just can’t get any further on this task. How should I create a cellar machine for this language? The word is asymmetrical for n = 1 and has a character length of 3. So the cellar will never be empty when the word has been processed. I hope you can help me.