You have to figure it out. First, the sum of the angles in a polygon = n-2(180) where n is the number of sides. For a pentagon, this is 5-2(180)=540. then divide by 5 for each of 5 large angles in the pentagon, and then divide by 2 for the angle formed by the radius in the right angle. This is 540/5=108 divided by 2 = 54 degrees in the right angle. Now that you know the angle , then use trig to solve the problem...