The general formula for this kind of sequence (arithmetic) is a(n) = a(1) + (n-1)c, where c is the common difference. Here, we have a(6) = -1 = a(1) + (6-1)(-3), or -1 +15 = a(1). The first term is 14.
The line would be called the median. The definition of '<span>A line connecting a vertex of a scalene triangle with the midpoint of the opposite side' pertains to the median. There are three medians that connect the scalene triangle. </span>