456 is smaller than 663 so it is closer to the 0 on the number line. When you subtract a number, you follow the number line to the left. When subtracting 663 from 456, it reaches 0 before it moves into the negative section of the number line. It takes 456, 456 steps to reach 0 and the answer is how many steps it needs to take after reaching 0 for the total steps to be 663 steps. This can be found by either continuing the movement on the number line or by simply doing 663-456, which is 207. Hence the answer to 456-663 is -207.
Hope this helps you!
