The equation is that of a pair of cones (or is it a single cone ?) with a common vertex and an axis of symetry along the axis. The cones open away from each other from the common vertex. The in the equation is the radius of the cone and the way that it is written says that it is a function of The radius will be zero when and when (or these coming from the generating line

What you are looking for then is a relationship of the form for some values of and such that and

That turns out to be and when you substitute that into the given equation it simplifies to your given answer.