Answer
S' is moving in the negative direction of the x axis.
Work Step by Step
We can find the velocity:
$v = \frac{\Delta x}{\Delta t}$
$v = \frac{480~m-1200~m}{5.00\times 10^{-6}~s}$
$v = -1.44\times 10^8~m/s$
Since the velocity is negative, S' is moving in the negative direction of the x axis.
