We see a radius of circle A labeled 3, so we know that the other radius of circle A that contains the side of the rectangle also measures 3 units. The side of the rectangle measures 2, so the part of the radius of circle A that is not part of the rectangle measures 1 unit. That is one leg of the right triangle.
Opposite sides of a rectangle are congruent, so the side of the rectangle opposite the side that measures 7 also measures 7. That is the other leg of the right triangle. Now we use the Pythagorean theorem to find AC which is the hypotenuse of the right triangle.
