By the Pythagorean theorem, the missing side is -sqrt7. The definition of sin is side opposite over hypotenuse, so go into Q2 and make an angle and label the side opposite the angle with a 3 and the hypotenuse a 4. The other leg is -sqrt7 cuz x is negative in Q2. The cosine of an angle is the side adjacent over the hypotenuse, so the cos then is -sqrt7/4