Just remember in Law of cosines:
The A there is the angle opposite of side a.
In the case of the formula given to you for triangle RST instead of A, S was used and that means the letter in the left side of the equation would be s, so S = 5.
If you look at the base formula given to you, 5 is S. With that alone you can eliminate the first three choices because none of them shows that S = 5.