If I am asked to find out MR at x=7 (where x is the quantity) and interpret it, how would I do it? Suppose, B is the result of MR at x=7. Do I interpret it as, "if the quantity changes from 6 to 7, the firm earns an extra revenue of B" or "if the quantity changes from 7 to 8, the firm earns an extra revenue of B"?