Answer
See answers.
Work Step by Step
The free body diagram shows that there is a downward gravitational force and an upward buoyant force on the fully submerged scuba tank.
Find the net force when the tank is full.
$$F_{begin}=F_{buoyant} -mg = \rho_{seawater}V_{tank}g-(m_{tank}+m_{air})g$$
$$ = ((1025\;kg/m^3)(0.0157\;m^3)-(17.0\;kg))(9.8\;m/s^2)=-8.89\;N$$
We see that the net force is downward.
Find the net force when the tank is empty.
$$F_{end}=F_{buoyant} -mg = \rho_{seawater}V_{tank}g-(m_{tank}+m_{air})g$$
$$ = ((1025\;kg/m^3)(0.0157\;m^3)-(14.0\;kg))(9.8\;m/s^2)=20.5\;N$$
We see that the net force is now upward.