2 feet below the ground level at the place where he started.
Step-by-step explanation:
After he adds 12 feet of elevation to the 6 feet above ground level where he launched, Mr Burnam is 6+12 = 18 feet above the ground level where he launched.
If he descends 20 feet, he will be 18 -20 = -2 feet above the ground level where he launched. That is, he is 2 feet below the ground level where he launched. We hope the ground slopes downward somewhat so Mr Burnam is not buried in the ground at that point.
