Since it's isosceles, ∠ABC = (180-30) / 2 = 75
You need the Law of Cosines here and you use it when you have 2 sides and an enclosed angle. The side across from the angle is the one we are looking for. The correct way to express the Law using what we have is the last choice above. Side RT is the unknown, and it is across from the angle that is enclosed between the 2 other sides.
