Seeing as it's for an assignment I'll not just blurt out an answer, but think of it like this:
Scratch's maximum resolution for an image is 480x360 - I.E. 480 wide and 360 across. To simulate a bigger screen, you must first make the background move in the inversely to the player. Second, you must place the other pieces of the background appropriately. I.E. Background 1: X:0 Y:0 Background 2: X:479 Y:0
Thus Background 2 looks like it's connected to background one. And if you move both simultaneously, then it looks like one big image, and moving inversely with the player, creates the illusion of scrolling.
This is a very, very basic version of a scrolling background. If you're planning for a smooth camera pan you'll need to set a center point and use the distance from the character/n to change the x position, and if you're looking for a more realistic look you'd have to take into account making a foreground, focal, and background which move at differing speeds, but seeing as all you've asked about is a simple scrolling background, I won't delve into that.
Hope this helps,
- iNK
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