Answer
The force on the south side of the box has a magnitude of $4000~N$ and this force is directed toward the south.
Work Step by Step
The pressure inside the box pushes outward in all directions on the box. We can find the force on the south side of the box due to the gas inside:
$F = P~A = (4.0\times 10^5~N/m^2)(0.10~m)(0.10~m) = 4000~N$
The force on the south side of the box has a magnitude of $4000~N$ and this force is directed toward the south.
