6.3
Looking at the triangle, you can see that point D bisects line segment AB and that point E bisects line segment BC. In fact, you can easily determine that triangles ABC and DBE are similar triangles with DBE having sides that are half the length of the respective sides in ABC. And since line segment DE corresponds to line segment AC which is 12.6 units long, line segment DE is 12.6/2 = 6.3 units long.
