Please attach clear photographs of effected area as without proper history and looking at problem it's difficult to advise you anything. You can consult with doctor online or offline for better guidance and treatment options.

Hello dear Please be calm I suggest you to please share pic for confirmation of diagnosis because it seems to be either a plunger cusp or loss of contact with type 3 embrasure. For further details you can consult me Thanks

Kindly visit a dentist in your area and get a dental scaling done.

I advice you to go for a professional scaling and polishing session.

Hello, Yellow stains are because of two reasons, it may be because of intrinsic stain or extrinsic stain. Intrinsic stains are because of genetic or environmental reasons like fluorosis which can be removed by teeth bleaching, laminates and veneers. Extrinsic stains are staining on the surface of teeth because of foods or drinks. Improper brushing also leads to plaque accumulation which causes deposition of calculus. It cant be removed by normal brushing or flossing. you need to visit dentist for scaling procedure which removes all deposits. A clinical evaluation is a must to come to proper conclusion