Radial component of the electric field?

A solid metal sphere of radius a= 1.30 cm is surrounded by a concentric spherical metal shell of inner radius b= 2.70 cm and outer radius c= 3.20 cm. The inner sphere has net charge Q1= 3.50 microCoulombs, and the outer spherical shell has net charge Q2= -8.50 microCoulombs. What is the radial component of the electric field Er at a point located at radius r= 2.40 cm, i.e. between the two conductors? (Er is positive if E points outward, negative if E points inward.)