A triangle ABC, in which we know the value of 2 sides, but the third side is unknown ( Side A ). If we need an angle to find the unknown side, which angle is it?
If you know the angle contained by the two sides with known lengths (that is the angle opposite the unknown side)
you can use the Cosine Law to determine the length of the third side.
Note that the angle you need is the one contained between the two given sides as demonstrated below: