If we connect the tips of the distances from the west and south, we form a right triangle with legs equal to 35 km and 72 km. The unknown shortest length is the hypotenuse. Using the Pythagorean theorem, we can say that,
h² = (35 km)² + (72 km)²
The value of h is equal to 80.06. Thus, the shortest length of the road that can be built is approximately 80.06 km.