Parabola problem about the sag of a wire

[bea995]

Homework Statement

Determine a function for wire between two poles if the arc of the wire is parabola. For every 75m there is 1m of sag.

Homework Equations

y = a(x-p)^2 + q

The Attempt at a Solution

I think that you need to make the lowest point of the wire the origin and hence the resulting equation that needs to be used is y=ax^2 but am unsure what to do from there.

 

[CompuChip]

You can make the lowest point the origin. Suppose the distance between the wires is $2d$, so that the end points of the wire are $x = -d$ and $x = d$.
Which three points of the parabola do you know now?