Answer: -231 ft
Explanation:
The submarine started at sea level = 0
It submerged 140 ft = -140 ft
It traveled 40 miles horizontally so it is still at -140 ft
It dove down an additional 120 ft = -260 ft (-140 + -120)
It traveled 25 miles horizontally so it is still at -260 ft
It rose 29 ft = -231 ft (-260 + 29)
Note: submerged and dove are negative values and rose is positive
0 - 140 - 120 + 29 = -231