Vivian is at a fair in Coachella Valley, California. Much of the valley lies below sea level. Vivian has brought along an altimeter to measure her elevation at different places in the valley. Vivian’s first ride was on the Ferris wheel. When she sat on the wheel at its lowest point, her altimeter read -9.6 feet. When she reached the top of the wheel, her altimeter read 12.3 feet. Complete the following steps to calculate the change in Vivian's elevation from the bottom of the Ferris wheel to the top.

Part A
Is the change in Vivian's elevation from the bottom of the Ferris wheel to the top a positive number or a negative number? Why?

Part B
Write an expression to represent Vivian's change in elevation from -9.6 feet at the bottom of the Ferris wheel to 12.3 feet at the top.

Part C
Subtracting a number is the same as adding its additive inverse. For which elevation do we need to find the additive inverse? What is the additive inverse of that elevation?

Part D
Now, rewrite the expression from part B using the additive inverse that you identified in part C.

Part E
Simplify the expression that you wrote in part D, and explain what the number means with regard to the problem.