The situation that describes opposite quantities that, when combined, would equal 0 is:
A diver was 30 feet below the surface and swam upward 30 feet. What is the change of elevation?
In this case, the diver's initial position of -30 feet (below the surface) and the movement upward of +30 feet result in a net change of 0 feet (−30 + 30 = 0).