Calculate Net Charge On Sphere Placed At Center Of Conducting Shell

Question

A solid conducting sphere of 15 cm radius is located at the center of a spherical conducting shell, whose inner radius is 21 cm and outer radius 42 cm. At a distance of 50 cm from the center of these concentric conductors, the electric field is 2.1 x 10⁵ V/m and is pointing outward. The shell has a net charge of  +3.1 µC. What is the net charge on the sphere at the center in µC?

(1) +2.7

(2) +1.8

(3) 0.0

(4) –1.2

(5) –3.1