Eric has made great progress in rehabilitating his knee with his physical therapist. The therapist measures the angle of Eric's fully bent knee as 45o. This angle is represented by ∠BCD in the diagram. Eric then extends his leg until it is completely straight, 180°. What is the full range of motion of Eric's knee, m∠ACD?

Help? Please, i been stuck on this nonsense for 15 mins now. （；´д｀)