Well it seems to me the process is fairly straight forward but the implementation could be complicated depending on how fancy you wanted to get.
If I were given this problem it would be broken down into:
1) Determine the position of the ball in each frame.
2) Use some sort of calibration of frame time and image scaling to convert that position to (x,y,t)
3) Fit a linear function to x=f(t) and a quadratic function to y=f(t)
4) Determine when y(t) = 0 and calculate x at that time
Step 1 is a good image processing project to do automatically but for the first go aground I'd probably just display the image and ask the user to click on the center of the ball.
See the tech note http://www.mathworks.com/support/tech-notes/1500/1508.html for your curve fitting options. I'd probably go with a simple least squares fit.
