Ok so descend means to go down and ascend means going up. With that in mind, you know what operations to use. You start at -170 and then climb up 72 feet. 170-72= 98. Now you drop -88. 98-88=10 go up 40 next 10+40= 50. Lastly, you go up by 50 feet. 50+50=100 which means your current elevation is 100 feet. I hope this helps you :)