User blog comment:John Lee Gamma Guild Team Vi Hart/Law of cosines/sines question???/@comment-25304012-20150827120926

You should use the law of sines when you know 2 sides and an angle (case 1 in the picture below) and you want to find the measure of an angle opposite a known side. Or when you know 2 angles and 1 side and want to get the side opposite a known angle (case 2 in picture below).

The Cosine Rule can be used in any triangle where you are trying to relate all three sides to one angle. If you need to find the length of a side, you need to know the other two sides and the opposite angle. Side a is the one you are trying to find.