Answer:
[tex]21.3\:\mathrm{mi}[/tex]
Step-by-step explanation:
The Law of Cosines is given as the following:
[tex]c^2=a^2+b^2-2ab\cos C[/tex].
Plugging in given values and solving, we get:
[tex]c^2=15^2+18^2-2\cdot 15\cdot 18\cdot \cos 80^{\circ},\\c\approx \fbox{$21.3\:\mathrm{mi}$}[/tex]
