Athin spherical shell with radius r1 = 4.00 cm is concentric with a larger thin spherical shell with radius r2 = 6.00 cm . both shells are made of insulating material. the smaller shell has charge q1=+6.00nc distributed uniformly over its surface, and the larger shell has charge q2=−9.00nc distributed uniformly over its surface. take the electric potential to be zero at an infinite distance from both shells.