Answer
Rae:
If your cats breath is that bad you should have his teeth looked at. There may be an infection, some teeth may need extracted and the rest scaled. If left untreated bad teeth can lead to other problems, a big one being organ damage. If there's already tartar buildup, no amount of brushing or treats is going to remove it.
Jack:
Bad breath in cats is due to bad teeth - same as humans. She needs a vet visit which will include a dental exam. Most cats actually need a dental cleaning done by age 5 or so.
Hope:
Kitty dentures like the commercials! Kidding, there are some cat snacks that are meant to better their oral health of all sorts. Doesn't have to be biscuits.
There are dental sticks, and even pastes you can sneak into her food. If it's terrible, take kitty to the vet and get Dr to do the dirty work of brushing kitty's teeth.
David:
Do you feed it wet food? If so you could try dry food instead. It leaves less food around the teeth which is the main source of bad breath.
