Hj and jk are the same length line segments ( because the midpoint divides a line into two equal parts) So hj = jk.
hk is the line segment which has the mid point j. It is the double of hj or jk. It can be the sum of hj and jk. hj + jk = hk or 2 * hj = hk or 2 * jk = hk
