## Precalculus (10th Edition)

The first thing I do is count any of the three unknown angles using the Law of cosines. By the law of cosines: $c^2=a^2+b^2-2ab\cdot cos(C)$, $b^2=a^2+c^2-2ac\cdot cos(B)$, $a^2=b^2+c^2-2bc\cdot cos(A)$, where $C$ is the included angle of $a$ and $b$ etc.