A mid-segment: is a segment connecting the half-sides of the other two triangle sides (median to both) and parallel to the base.
we can say that the two triangles are similar the one with the mid-segment as base and the one with the base as base
0.5AB/AB = mid-segment/base
mid-segment = 0.5 base = 0.5 x 21 = 10.5 inches
