Final answer:
The 'change x by' block is used in Scratch to move a sprite left or right on the x-axis, with negative numbers for left and positive numbers for right.
Step-by-step explanation:
In Scratch, the block used to change a sprite's x-position (to move left or right) is the "change x by" block. If you want to move a sprite to the left, you use a negative number with this block, and for moving it to the right, you use a positive number. For example, if you want to move a sprite 10 steps to the right, you would use the block with "change x by 10". If you want to set the sprite's x-position to a specific value, you would use the "set x to" block.