Let's consider the following two energy profile diagrams:
Notice how the endothermic reaction has to overcome the energy of deltaH AS WELL AS a further little bit? Then, for the exothermic reaction, it only needs to get over that little bit?
So, let's call that little excess bit q, as just some random bit of energy. From the energy profile diagram, we can see that:
And from this, we get:
Since you know the activation energy for the endothermic reaction, to find the activation energy, you either use the above formula to find the activation energy for the reverse, or exothermic, reaction. However, I do suggest logicing it from energy profile diagrams rather than trying to memorise this formula.