So if the noun was masculine you would use तुम्हारे, but because it's feminine we use तुम्हारी and it doesn't matter that it's plural, right?
Exactly! You're on the right track. Enjoy DuoLingo :)
Are but is
" गाड़ियाँ" - can be vehicles right. Not just cars
achhi also means nice, so that should be accepted.
Your cars are good