The distance SA + SB will be minimized.
If the station is x units away from the west end, then it will be 200 - x units away from the east end.
Use the right triangles:
SA^2 = x^2 + 90^2
SB^2 = ( 200-x)^2 + 60^2
Solve for SA and SB above, respectively, then add both sides to get total length SA + SB = L(x)
Optimize L(x) = using calculus.
I hope this helps. Good luck!