You must mean the depth of the submarine.
to make sum = zero the depth of the sub must be -1600 ft..
5^-3 is 1/125
The negative in the exponent makes it so there is a 1 over the problem if there was no negative. When you have a negative in the exponent, the answer will be the reciprocal (opposite) of the question with no negative.
For example,
9^2 = 81
9^-2 = 1/81
48/75 = 0,64 and they want a percentage so 64%
