Answer: Hello mate!
If we consider y = 0ft in the surface (the initial position of the submarine)
When the submarine does the first dive, it descends 375ft
then the position is y = 0ft - 375ft = -375ft
where the position is negative because the submarine is descending.
Now, the submarine decends another 175ft, so the new position is:
y = -375ft - 175ft = -550ft
