Chapter 1 - Foundations for Algebra - 1-5 Adding and Subtracting Real Numbers - Practice and Problem-Solving Exercises - Page 34: 42

20

The bird starts out at 42 feet. That is where 42 comes from. The bird then goes up 144 ft, so, +144 ft. It then goes down 126 ft, so, -126 ft., It then goes back up for 25 ft., so, +25 ft., and then it goes back down to land on the brook, down 65 ft, so, -65 ft. Adding and subtracting all of these gives you an equation, which is 42+144-126+25-65=20, which means the brook is 20 ft. above sea level.