A thick shell with inner radius R and outer radius 3R has a uniform charge density σ coulomb/meter cube it has a spherical cavity of radius R as shown in the fig. above,
1use superposition of electric fields....
take the center of the spherical cavity of radius R as point p
E1 field at p due to the whole thick shell can be calculated by gauss law
this E1 = Ereqd + E2 [E2 being the field at pnt p due to the spherical cavity only,so E2=0 by gauss law,u can use symmetry as well]
find E1....u`l get the answer....