Answer
$$\theta \approx 130.5 ^o$$
Work Step by Step
$\theta = \angle C$
$$cos \space \theta = \frac{a^2 + b^2 - c^2}{2ab}$$ $$cos \space \theta = \frac{(12)^2 + (10)^2 - (20)^2}{2(12)(10)}$$ $$cos \space \theta = 0.65$$ $$\theta = cos^{-1} \space -0.65 \approx 130.5 ^o$$
