You know three points on the parabola: (0,0), (150,30), (-150,30) (I’m guessing about the 30. The picture is not clear enough on my phone.)
The parabola is vertical, so its equation has the form y = a(x-h)^2+k, where (h,k) is the vertex.
The vertex is (0,0), so y = ax^2.
Plug another point into the equation and solve for a:
30 = a150^2
a = 30/150^2 = 1/750
y = Height of cable at a point x feet from center.
y = (1/750)x^2
