A disk has radius R = 2.90 cm. Its surface charge density is 1.70 10-6 C/m² from r = 0 to r = R/2 and 9.50 10-7 C/m² from r = R/2 to r = R. (a) What is the total charge (in C) on the disk? (b) With V = 0 at infinity, what is the electric potential (in V) at a point on the perpendicular central axis of the disk, at distance z = R/2 from the center of the disk?