Answer:
17.5 m
Step-by-step explanation:
The height of a parabola above its vertex is proportional to the square of the distance from the vertex.
The towers are 1200 m apart, so each is 600 m from center. The location of interest is 300 m from center, so 1/2 the distance to a tower. Since the height is proportional to the square of the distance, the height half-way to the tower will be (1/2)² = 1/4 the height of the tower.
(1/2)²×(70 m) = 17.5 m
The height of the cable 300 m from center will be 17.5 meters.