First, let's make a diagram to visualize the problem.
We know that, at the start, runners are 13.8 kilometers apart. So, the distance of runner A is going to be x, while the distance of runner B is going to be 13.8-x. Then, we use the equation of constant motion.

Let's express an equation for each runner using the given velocities and the expressions of the distances we defined before.

Now we combine the equations to solve for x.
![\begin{gathered} (x)/(2.6)=(13.8-x)/(8.6) \\ 8.6x=2.6(13.8-x) \\ 8.6x=35.88-2.6x \\ 8.6x+2.6x=35.88 \\ 11.2x=35.88 \\ x=(35.88)/(11.2) \\ x\approx3.2\operatorname{km} \end{gathered}]()
The distance of Runner A is 3.2 km.
Then, we subtract to find the position with respect to the flagpole.
![8.4km-3.2km=5.2\operatorname{km}]()
Therefore, they will be 5.2 km west.