If we were adding two positive integers like 5 + 8, how would we show this on the horizontal number line? We’d start at 0 and move 5 units to the right; then, we’d move another 8 units to the right to end up at the sum of 13. To add a positive integer, we move right, the positive direction, on the number line. If it were a vertical number line, we’d move up.

To add a negative integer, we do the opposite: we move left, the negative direction, on the number line. If it were a vertical number line, we’d move down.

How would we add -3 + (-9) on the number line?

A
moving 3 units to the left, and then another 9 units to the left

B
moving 3 units to the left, and then 9 units to the right

C
moving 3 units right, and then 9 units to the left

D
moving 3 units right and then 9 more units to the right