The background plane will feature a much simpler animation, with a simple move from left to right. As a result, we won't need any extrapolation movement.

Here are the frames and actions required:

• Frame 120: Move the background to the left, until it almost reaches the right border of your camera. Press the I key and add a location keyframe.
• Frame 1: Slide the background to the opposite direction and make it get close to the left border of the camera frame. Press the I key to add another location keyframe.