By definition, when an object is reflected twice over parallel lines, the distance between the points of the original object and the the point of the final translation of the object, is twice the distance between the two parallel lines. Keeping this information on mind, you have:
The distance between points of the original and final translation image is 26 centimeters. Therefore:
2d=26 cm
d=26 cm/2
d=13 cm
Therefore, the answer is: 13 cm.
