I don't know how else to phrase this problem. I am really curious what are the steps to solving this problem.
In case you did not understand what the problem is about, I'll try to explain better: There is a horizontal segment of known length and at its sides there's two perpendicular segments(poles) of known lengths as well. I need to find a point on the horizontal line such that the distance from the top of the poles to x are equal.
Let the point $p$ divides the segment in two parts with lengths denoted by $x$ and $l-x$
We denote the length of poles by $a$ and $b$
You construct two right triangles whose hypotenuses should be equal.
Thus $$a^2+x^2=b^2+(l-x)^2$$ Solve for $x$ to get $$x=\frac {b^2+l^2-a^2}{2l}$$