Noticed a sore that's on the gum of my back left molar. doesn't burn or anything. also a somewhat raised white line on gums of cheek. what can it be?

Lots of things...: This could be trauma from chewing or brushing, a foreign body stuck in your tissue, infection in the tooth or gums, scarring, or neoplasm. If it hurts, bleeds, grows, or lasts more than a week or two: you should have it professionally evaluated. These type of things can be nothing to worry about OR life threatening; that's too much wiggle room to sort out online.

Sore on gum: It could be any number of things...tooth related, gum related, infection, etc. Impossible to make a diagnosis without a visual and clinical examination. Make an appointment to see your dentist.