I have a black line in my nail that has been there for about 7 yrs. saw 2 derma already and both said it does not look like cancer but a mole. biopsy?

No need : An issue that has not changed in 7 years is not a cause for concern. You could make matters worse by not leaving it alone. Get HPV and other vaccines.

Line on nail: Black line underneath nail is called a splinter hemorrhage. It occurs for a variety of reasons and may be harmless or sign of a more serious condition: in absence of trauma- infections, vasculitis, nail fungus, lichen planus, meds that alter blood clotting, kidney, lung, or circulatory diseases, chemotherapy. Melanoma. See doc -

Direction?: If line goes from side to side it may be known as Beau's Lines which are relatively benign. If the line starts at the beginning of the nail and travels to the end, a portion of the part at the end may be snipped and sent off for analysis. Podiatrists see many of these cases and may be able to recognize it instantly. Watch out for changes in appearance or pain and report changes to your doctor.

No biopsy: 7 years of an isolated black line without evolution= benign nevus. Forget it unless something changes.