Hi Lawrence, Both are correct. "A certain" implies that the noun following it is singular, so as long as there's a singular noun, you're fine. "Certain" without an article before it implies that the noun following it is plural.

Yes, you can say "a certain car" to refer to a specific car, and also "certain cars" to refer to a certain distinct group of cars, for example: "Certain cars are more fuel efficient than others." Hope this helps