To do this, we can use the Law of Cosines, which states that for any triangle with sides $a$, $b$, and $c$, and angle $C$ opposite side $c$, we have:
$c^2 = a^2 + b^2 - 2ab \cos(C)$
Plugging in the given values, we have:
$c^2 = (21.9 \mathrm{km})^2 + (24.6
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