"The jacket of the boy."
Translation:Το μπουφάν του αγοριού.
It's not in my dictionary, but ζακετα is. (My keyboard doesn't do accents.)
You may want to read the comments in this discussion: https://www.duolingo.com/comment/17496123 I suggest doing an image search, it's very reliable.
What keyboard layout do you have? On a standard QWERTY, accents are added by hitting first the key next to L, then the vowel (; or : + vowel): ;+α=ά.