How do I gradually slow down a scrolling image to a complete stop?

Hi! I'm a total newbie when it comes to AE.

As part of a school play I'm overseeing, I've been asked to scroll a backdrop across a screen as a car drives along and then slowly comes to a stop. I've figured out how to make the backdrop scroll in a seamless loop, but I have no idea how to make it gradually come to a stop.

How would I go about this?

Thanks,

Adam