The surface of the water is at 0 meters.
Ben is 15.8 meters below the surface of the water.
if he dives down the same distance again then
15.8 meters + 15.8 meters = 31.6 meters below the surface
Then moved 12.4 meters towards the surface (means opposite direction so we need to subtract this distance)
31.6 meters - 12.4 meters = 19.2 meters below the surface
Therefore, the new position of Ben will be 19.2 meters below the surface.