Explain how to determine whether to move right or left on the number line when adding rational numbers

If you're adding a negative number you move to the left, because that's where negative numbers are. if you're adding a positive number you move to the right.

example:

-4 + 7

you move seven spots to the right, since 7 is positive the result is 3

4 + - 5

move 5 spots to the left, since 5 is negative, the result is - 1.