See below.

First make a diagram of the situation as shown: We are looking to find the distance RT.
The Cosine Rule states, for a triangle ABC.
a2=b2+c2−2bc⋅cos(C)
Making C=S=120∘
We have:
(RT)2=(ST)2+(RS)2−2(ST)(RS)⋅cos(S)
(RT)2=2002+1502−2(200)(150)⋅cos(120∘)
So the first one is the correct solution.
