To find side $ c $ use Law of Cosines:
$$ c^2 = a^2 + b^2 - 2 \cdot a \cdot b \cdot \cos( \gamma ) $$After substituting $a = 9$, $b = 15$ and $\gamma = 66^o$ we have:
$$ c^2 = 9^2 + 15^2 - 2 \cdot 9 \cdot 15 \cdot \cos( 66^o ) $$ $$ c^2 = 81 + 225 - 2 \cdot 9 \cdot 15 \cdot \cos( 66^o ) $$ $$ c^2 = 306 - 2 \cdot 135 \cdot \cos( 66^o ) $$ $$ c^2 = 306 - 270 \cdot 0.4067 $$ $$ c^2 = 306 - 109.8189 $$ $$ c^2 = 196.1811 $$ $$ c = \sqrt{ 196.1811 } $$$$ c \approx 14.0065 $$