There's some general rule about it, or is something irregular and changes in every case?
The rule of thumb for nouns in plural form that end with ות- is that this suffix remains in construct state as well.
(paper - masculine): נייר (paper) - ניירות (pieces of paper) - ניירות שעווה (pieces of wax paper)
(meal - feminine): ארוחה (meal) - ארוחות (meals) - ארוחות צהריים (lunches)
Who ever calls them "safety belts"? I never hear anyone call then anything other than "seat belts" in both cars and planes.
In Canada both are used commonly for planes, but more commonly used seat belts for cars
אין במכונית הזאת is " there are not IN this car" אין למכונית הזאת would be "this car does not have". The meaning is the same so BOTH should be accepted. Reported.
My answer 'This car has no seat belts' was marked wrong. It means exactly the same in English as 'This car does not have safety belts', the given answer. (Reported)
That's because it's a truck in the shape of a car (SUVs) and, as DL taught us earlier, trucks don't need "safety" belts ;-)