READ: This was quickly made to explain. Solution: You are trying to find the length of the red line, since that is the line that starts at the topmost point and ends at the rightmost point. The green lines are both 2, since the radius of the circle is 2. These 3 lines form a triangle, and the hypotenuse can be solved by the Pythagorean Theorem. 2 squared plus 2 squared is 8, so the red line is √8.
Made for a riddle I asked on piyj's "test" project.