1. The problem is to find the net elevation change when moving 800 feet below, then 450 feet above, then 100 feet below, and finally 210 feet above. 2. Represent the changes as signed values: going below means negative and going above means positive. 3. So, the expression is $$-800 + 450 - 100 + 210$$. 4. Calculate the sum step-by-step: $$-800 + 450 = -350$$ $$-350 - 100 = -450$$ $$-450 + 210 = -240$$ 5. The net elevation change is $$-240$$ feet, which means 240 feet below the starting point.