You don’t need maths for dentistry,
If you’re struggling with maths now, your best choice would be to drop it, as it gets progressively harder.
Since you got a 6 at gcse, it might just mean you need to work a little harder than anyone else but you could still get an A at A level if you worked hard. I know someone that got a 6 at gcse and now he’s working at an A* so it’s definitely achievable.