Leon is standing on a platform that js 3 feet above the surface of a pool he jumps into the pool and reaches a depth of 6 feet below the surface

He went down 9 feet, aka - 9.

Step-by-step explanation:If he is 6 feet below the surface then he is in the negatives, - 6, all you need to do is add the numbers to see how much he went down, you need to reach the original number, like - 6 (current) plus + 6 (the opposite of his negative to reach the normal surface ground), plus 3 (where he was before) equals 9.