Two point charges Q and-3Q are placed at some distance apart. If the electric field at the location of Q is E, then at the locality of -3Q, it is

The field produced by charge -3Q at A is E as mentioned in the question.∴E=3Qx2(along AB directed towards negative charge)Now, field at location of -3Q, i.e., field at B due to charge Q will be E′=Qx2=E3(along AB directed away from positive charge)