Graph parabola in flash?

[flashpipe]

I need to create a parabola in flash where the user can adjust 3 sliders which control the vertex and the two x-intercepts. I tried using the parabola code at:
http://www.flashkit.com/movies/Scrip...1568/index.php
but that doesn't let you adjust the x-intercepts. So, I created a version:
http://www.brianwpiper.com/gridParabola.fla
but I can't figure out how to extend the lines across the grid. I tried calculating a new x and y at either extreme and then drawing lines to them, but I can't get the correct equation to properly calculate the points.

Anyone have any idea how I might do this or where I might get some other ideas??

Thanks!!!