The third side

Given, two adjacent sides of a triangle having lengths, a=6 and b=4.

Their included angle is A=60 $^\circ$ .

The side opposite angle, A has length, c.

Find c $^2$ .

Using the "Law of Cosines," $c^2 = a^2 + b^2 -2ab \cos A$ ,

$c^2 = 6^2 + 4^2 - 2 \times 6 \times 4 \times \cos 60^\circ = \boxed{28}$