How do I set boundary conditions for my bowling game?

I'm making a bowling game on MATLAB. So far I have set the images for the background and pins. How do I make the ball roll down the side alley in case it goes near the edge? I made equations to compare x values at a fixed y value but it has no effect. What would you suggest?

[Walter Roberson]

Please show your code attempt, the one that tries to compare but does not work.