We must decompose the displacement vector to find the new displacement to the west.
\(d_{x}=v.cos\O \\ d_{x}=200. \frac{1}{2} \\ d_{x}=100km\)
Adding all displacements to West, It comes:
\(\Delta S= d_{o}+d_{x} \\ \Delta S= 150+100 \\ \boxed {\Delta S= 250 km}\)
If you notice any mistake in English, please let me know, because I'm not native.