An endothermic reaction $\mathrm{A}⟶\mathrm{B}$has an activation energy as $\mathrm{xKJ}.{\mathrm{mol}}^{-1}$of A. If energy
change of the reaction is  y KJ, the activation energy of the reverse reaction is

For an endothermic reaction, the activation energy for the backward reaction is equal to the difference of the activation energy of the forward reaction and the enthalpy change of the reaction.

Thus, the activation energy of the reaction B→A is x-y kcal.