Answer:
 below sea level. 
Explanation:
We have been given that a submarine hovers at 66 and 2/3 yards below sea level. This means that the position of submarine is negative.
We can represent this information as:
 
We have been also given that the submarine ascends 24 and 1/8 yards. This means we need to add 
 to 
.
 
 
 
 
 
The submarine descends again 78 and 3/4 yards. This means we need to subtract 
 from 
.
 
 
 
 
 
 
Therefore, the submarine's new position is 
 below sea level.