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

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

90 feet above and then we descend 105 feet, meaning we subtract:
90 - 105 = -15.

We read this 15 feet below ground level