One of my cats has plaque on his molars and no matter what I do I can't get rid of it?

If your cat will stay still for it, you can chip the tartar off, with your fingernail if you have strong nails, with a dental instrument otherwise. But it can be hard to do with cats since they have such small teeth and naturally don't enjoy having this done. Best bet might be to have the vet do the cleaning, and then maintain the teeth after that. One of my cats is on a special dental food which is supposed to prevent tartar. Some animals are more prone to tartar build-up than others. I've had dogs who ate exactly the same thing, one got tartar and the other didn't.