The parabola is 1600 meters wide. Since a parabola is symmetrical it extends 800 meters to right and left. At 800 meters to the right or left, the height is 60 meters.

So, the parabola must be

y = kx^2

60 = k(800^2)

k = 60/640,000 = 6/64,000 = 3/32000

y = 3/32000x^2

Then, at 400 meters, the height of the cables is found using that equation: