Answer
240.76 pm
Work Step by Step
The hexagon has all internal angles of $120^{\circ}$ and sides of length 139 pm.
Labeling the vertexes from 1 to 6 in a counterclockwise manner starting at the top, we want to know the length of the line connecting 2 to 6.
Projecting 1 into this line we get a point 1$^*$ defining the right triangle $1-2-1^*$, where the angle $\widehat{2-1-1^*}$ is $60^{\circ}$ (half of the internal angle). So the length of 2-1$^*$ is given by:
$sin\ 60^{\circ}=\sqrt 3/2=\overline{2-1^*}/139\ pm$
$\overline{2-1^*}=120.38\ pm$, which is half of the length we want, so the answer is 240.76 pm
