The elevation we call sea level is actually the average level of the sea. This means that at average sea level, the elevation is zero. In reality, the level of the sea rises and falls in what are called tides. Low tide is the point of lowest sea level in a tidal period. High tide is the point of highest sea level in a tidal period. The height above or below average sea level is called elevation. An elevation below sea level is considered a negative value, while an elevation above sea level is positive.
Winston lives near the ocean. He regularly brings an altimeter to the beach to measure the elevation of the ocean’s surface. You will use the data Winston gathered to complete tasks about the rising and falling of the ocean’s surface. When necessary, sketch a diagram of the situation on a separate sheet of paper to assist you.
On Monday at 1pm, when the ocean’s surface was at an elevation of 2.2 feet, Winston went to a surf shop that was 8.2 feet above the ocean’s surface. At 7 p.m. when the ocean’s surface was at an elevation of -2.4 feet, Winston ate dinner at a restaurant whose deck was 10 feet above the ocean’s surface.

What was the net change in the elevation of the ocean’s surface from 1 p.m. to 7 p.m.?

The question is asking for the net change in the ocean's surface elevation, so we can ignore the parts of the question that mention his own elevation at the time of the measuring. We can see at 1, the elevation was 2.2, and at 7, the elevation was -2.4. The net change is the whole change over the period of time, so we just need to find the difference between the two. Adding them together gives you the distance from each elevation the sea's surface was, which is -4.6 ft as the elevation decreased