Final answer:
To calculate the coaster's height after descending 105 feet from an initial height of 90 feet, subtract the descent from the initial height, which results in -15 feet, indicating the coaster is below the starting ground level.
Step-by-step explanation:
To find the height of the roller coaster after the first descent, we subtract the distance it descends from its initial height above the ground level. In mathematical terms:
Initial height = 90 feet
Descent = 105 feet
Final height = Initial height - Descent
Final height = 90 feet - 105 feet
Final height = -15 feet
Since the final height is below ground level, we express this as a negative number to indicate that the roller coaster is 15 feet below the starting ground level. Therefore, the correct answer is (c) -15 feet.