This question is basically asking what is -60 plus 25. It said that -60 is less than its previous altitude which means that we need to add to get the previous altitude. When you add a negative to a positive number all you do is count backwards just like you were subtracting. -60 + 25 equals -35. We can check this by subtracting 25 and -35 to get -60.
The submarine previous altitude is -35.
