If you are 2.25 feet above water and then dive directly down 3.75 into the water, how deep in the water will you be? explain.

1.5 deep.

you subtract 2.25 of air space from the total depth 3.75

3.75-2.25=1.5

The answer would be 1.5 feet.
To solve it, you take 3.75 feet and subtract 2.25 feet; so basically you just needed to find the difference between the heights.
The reasoning behind this, is you are starting at 2.25 feet, and then going down 3.75 feet. So if 0 represents the very top of the water, then going down 3.75 ft from 2.25 ft. would be -1.5. What this means in terms of depth is that you'll be 1.5 ft below the surface of the water, or put more simply: you will be 1.5 feet deep in the water. 

The answer is 9.7 cm