Ascuba tank, when fully submerged, displaces 15.7 l of seawater. the tank itself has a mass of 13.7 kg and, when "full," contains 3.00 kg of air. the density of seawater is 1025 kg/m3. assume that only its weight and the buoyant force act on the tank.

The problem is asking for the buoyant force.
The first step is to know the weights of the tank and air. Tank = 13.7 * 9.8 = 134.26 N Air = 3 * 9.8 = 29.4 N Total weight of the tank and air is 163.66 N 
To know the buoyant force, we must convert 15.7 liters to cubic meters. 
1 liter = 1000 ml = 1000 cm^3 1 m = 100 cm 1 m^3= 1,000,000 cm^3 One liter = 1000 divided by 1,000,000 = 0.001 m^3 V = 15 * 0.001 = 0.015 m^3 
Then compute for the buoyant force:Buoyant force = 0.015 * 1025 * 9.8 = 150.675 N