Congratulations! Did you visit your dentist? Your dentist can remove the hardened discoloration. Sometimes plaque can make your teeth appear really yellow. If you take some baking soda, add a pinch of salt, you can add a few drops of coconut oil and brush your teeth. Some patients use a few drops of lemon essential oil. When you mentioned salt some patients go overboard. Some brush hard and irritate their soft tissues. The oil can prevent the plaque from attaching to your tooth surface. Plaque just doesn't stay at the gum line, it can cover your entire teeth. The problem with salt and baking soda is there is so much of it. Down size your containers. This way you can control the amounts. Use a soft toothbrush. A hard toothbrush and brushing hard will damage the tissue around your teeth. That TISSUE IS PRECIOUS. WHEN YOU SEE YOUR DENTIST ASK HOW YOU SHOULD FLOSS AND BRUSH CORRECTLY. ASKED TO BE SHOWN. YOU WILL BE SHOWN WHERE PLAQUE AND FOOD COLLECTS. THE KEY IS TO FLOSS AND BRUSH CORRECTLY DAILY.