Chapter 8 - Applications of Trigonometric Functions - 8.3 The Law of Cosines - 8.3 Assess Your Understanding - Page 540: 59

Count the length of the third side using the Law of cosines.

The first thing I do is count the length of the third side using the Law of cosines. ($c^2=a^2+b^2-2ab\cdot cos(C)$), where $C$ is the included angle of $a$ and $b$.