The fact that the terminating side has 2 negative values tells us that it lies in Q3, where both x and y are negative. The cosine of an angle is the side adjacent/hypotenuse. But we don't have the value for the hypotenuse, so we have to find it using the Pythagorean Theorem. c^2 = -20^2 + -21^2 which gives us a c value of 29. Now that we have the hypotenuse length, we can set up the angle's cosine value: cos theta = -20/29.
