When can the law of sines be used?
The minimum data you need to solve a triangle is 3 between sides or angles, with the only exception of the 3 angles.
The Sines Theorem and the Cosines Theorem are "complementary". If you can use one, you can't use the other one. The Cosines Theorem can only be used in the case of having 2 sides and the angle between them. In ALL the other cases you have to use the sines Theorem.