A plastic rod has been bent into a circle of radius R = 6.30 cm. It has a charge Q1 = +5.00 pC uniformly distributed along one-quarter of its circumference and a charge Q2 = -6Q1 uniformly distributed along the rest of the circumference. Take V = 0 at infinity. What is the electric potential at the center C of the circle? What is the electric potential at point P, which is on the central axis of the circle at distance D = 6.71 cm from the center?