An early submersible craft for deep-sea exploration was raised and lowered by a cable from a ship. when the craft was stationary, the tension in the cable was 7000 n. when the craft was lowered or raised at a steady rate, the motion through the water added an 1800 n drag force.

The tension in the cable when the craft was being lowered to the
seafloor = 7500 N - 1800 N = 5700 N

I think you gave an incomplete question. The two questions would be:

A.What was the tension in the cable when the craft was being lowered to the seafloor? 

B.What was the tension in the cable when the craft was being raised from the seafloor?

The details for answering the question are already given.

A. Tension in the cable when
 the craft was being lowered to the seafloor = 7000 - 1800 newton
                                                                      = 5200 newton

B. Tension in the cable when
 the craft was being raised from the seafloor = 7000 + 1800 newton
                                                                       = 8800 newton.

The masses of the objects and their densities