Whats the difference in using accha and acchi?
You use 'accha' for masculine singular nouns, 'acche' for masculine plural nouns and 'acchi' for feminine (both singular and plural) nouns.
गाड़ियाँ is a feminine noun so we need to use 'acchi'.
Why was तेरी गाड़ियाँ अच्छी हैं not accepted as a solution? I do not see anything in the English translation that indicates "tum" instead of "tu" as the subject.
Should be accepted. You can report if you see the sentence again.