Two perpendicular segments form a 90° angle when they intersect, a 90° is usually depicted with a square. In this case, segments CD and GH form a 90° angle, then CD and GH are perpendicular lines, since AB and CD are parallel GH is also perpendicular to AB.
Then, AB is perpendicular to the line segment GH
E and F are at a distance "a" from each other, since the two segments where these points are located are parallel to each other any pair of points that lay on these lines are separated by a distance "a". G is on AB and H is on CD.
Then, the length of GH is a units