Background assets._x moving when player._x is moving (but with a lower speed)

Hello! Making a game and i want to create some depth in the background.

I have 1 background for the game.

Infront of that background i have a "middle background" with trees and stuff.

And infront of that background i have the plane where the character is walking.

I want to make so whenever the characters._x is either raising or getting lower then the background assets (the middle background) will also move in the same direction but with a lower speed than the character.

Any kind soul out there that can figure out the code for that?

I'm using Actionscript 2.0 :)