The line AC is one diagonal of the rectangle. As it can be seen form the illustration the other its diagonal BO is exactly equal to the radius of the circle, i.e. OC plus CD, 6 plus 4, or 10 units. Since the diagonals of a rectangle are equal, AC is 10 units long too.

Using the Pythagorean Theorem it can be figured out that the side BC is 8 units long (10² - 6² = 8²).