To prove that point B(7,4) lies on circle A, Riley should determine if the distance from the center of the circle (A) to point B is equal to the radius of the circle. The appropriate way to do this is by using the distance formula.
Thus, the best first step for Riley would be:
B. Riley should use the distance formula to calculate the length of segment AB.
If the calculated distance is equal to the radius (3), then point B lies on the circle.