You shouldn't just add things because someone tells you to; you should add them when they're a requirement.
As your code is right now, it simply constructs your Cars
(keep in mind that an object is typically singular, not plural) object through the constructor. If you never want to be able to change the properties of a car after it has been created then you're fine as it is.
A setter is there to provide manipulating access to the backing field. If you don't need or want this, then you shouldn't add it.
Sidenote: it might still be a good idea to add it but make it private
instead. If you want validation logic (for example to check that the price is not negative), then this should be done through a setter.