Changing the sign of the moving term before moving from one side of the equation to the other can be understood as follows: According to the law of subtraction: a-b=a+(-b), that is, subtracting a number is equal to adding the reciprocal of this number. When you want to move an item (such as X) on the left to the right, you actually subtract the item (X) on the left. According to the same solution principle, you must also subtract this item on the right. Then according to the law of subtraction, the inverse of this term (x) must be added to the right. Therefore, after the item (x) on the left is subtracted (from nothing), its opposite number will appear on the right (from nothing). It feels like the item on the left has changed its sign and moved to the right. It is the same reason to move some terms on the right side of the equation to the left.