A roller coaster begins at 90 feet above ground level. Then it descends 105 feet. Find the height of

A roller coaster begins at 90 feet above ground level. Then it descends 105 feet. Find the height of the roller coaster after the first descent.

90 feet above ground level is written as +90
Descending 105 feet means we subtract 105 feet to get:
+90 - 105 = -15 or 15 feet below ground level