(9)

When you add and , you’re essentially combining a positive number and its negative counterpart. This is like having twelve apples and then giving them all away — what do you have left? That’s right, zero! So, the box would be filled with .

On a number line, starting at and moving units in the negative direction, you land right back at . It’s a perfect balance; the positive and negative numbers cancel each other out to reach a neutral state!