I would orient my coordinate axes such that the vertex is at the origin. And so we know:

y(0)=0

y(±50)=10

A parabola opening upwards, with its vertex at the origin can be written:

y=ax^2 where 0<a

Now, using the point (50,10), can you determine the parameter a?

Once you do this, then evaluate y(25)+40 to find the height of the wire above the ground at X.

edit: Normally I like to use LaTeX, but it is not working at the moment.