Parallel lines have the same slope.
Find the slopes of segments DE and AB and show they are equal.
Slope of DE:
(y1/2 - y1/2)/((x1 + x2)/2 - x1/2) = 0/(x2/2) = 0
Slope of AB:
(0 - 0)/(x2 - 0) = 0/x2 = 0
Both segments have slope 0.
Since the slopes are equal, the segments are parallel.