Step-by-step explanation:
Let the surface of the lake = 0 feet
beneath the surface of the lake means we are subtracting
96 feet beneath the surface of the lake = 0 - 96 = - 96
He went up by 49 feet:
Going up means we are adding
The final position = -96 + 49
The final position = -47 feet
-47 feet means the final position is 47 feet below the surface of the lake
His depth is -47 feet or 47 feet below the surface of the lake