Chapter 1 - Introduction to Algebraic Expressions - 1.6 Subtraction of Real Numbers - 1.6 Exercise Set - Page 50: 133

950 m

The elevation starts at +550 m (since it's ABOVE sea level) and ends at -400 m (since it's BELOW sea level). The elevation decreases, so the number of meters the elevation falls is positive. The number of meters the elevation falls is the positive difference between the starting and the ending elevation, which is: 550-(-400) =550+400 (two negative signs make a positive) =950 m.