Generate parabola?

[flashpipe1]

I need to create a parabola between 3 points which are controlled by a slider:
the two points where the parabola crosses the x plane
the y location of the vertex of the parabola

I've tried adapting the code in:
http://www.flashkit.com/movies/Scrip...1568/index.php
but that doesn't let you set the x points.

I've got the code to set the location of the 3 points, but not sure what formula/convention I can use to draw the parabola between the points and have the ends extend across the grid?

Anyone have any idea what formula I might use?

See the attached file to see what I'm talking about...