To answer this question, first we need to understand the definition of a median of a triangle.
A median of a triangle is a line segment drawn from a vertex to the midpoint of the opposite side of the vertex.
AB is a segment drawn from the vertex A, to the point B, but B is not the midpoint of the base of this triangle(the midpoint divides the segment into two equal parts, and since one part is 7 and the other is 8, B is not the midpoint