We are looking for the distance between Start and End. In order to do this, we can create a right triangle from start to end, and solve for the hypotenuse.
The triangle will have side lengths 4, 5, and x, the hypotenuse/distance between start and end.
With the pythagorean theorem, sqrt(16 + 25) = x,
x = sqrt(41)
So once we round sqrt(41), the nearest tenth of a km is 6.4 .
