Disagree that "де є авта?" is an incorrect response. "авта" is a perfectly good translation of the word cars. Also, getting frustrated by the constant incorrect result for using the work "є".
I think авто is an indeclinable word in Ukrainian, like бюро, кіно, метро, so the forms are the same in all cases whether singular or plural.
Nowadays there is a tendency for this word (авто) to become declinable, however it's not a standard yet