Usually, we use positive numbers to represent "up", and negative numbers to represent "down." Then the description of the math might be ...
The change in height between the launch point on the cliff and the distance above the cliff that the rocket reaches is best represented by a <u> positive </u> number.
The change in height from the top height of the rocket to the ocean below the cliff is best represented by a <u> negative </u> number.
The expression <u> 16.8-28.1 </u> best represents the change in height from the cliff to the ocean below.
The change in height from the cliff to the ocean below is <u> -11.3 </u> feet.