What number should be added to -28 to make the sum 0

If you imagine a number line where 0 is in the middle, and all the negative numbers are on the left side of 0 and all the positive numbers are on the right side of 0, -28 will be 28 units across on the left side.

Therefore, to get back to 0, you need to go back 28 units (moving to the right), which is positive. Therefore, to make the sum 0, you need to add 28 to -28.