‘a’ is the energy change.
‘c’ is the activation energy
‘e’ is energy of products
Here, the energy of products is more than energy of reactants.
Therefore, (products – reactants) would give a positive value. Here it is labelled ‘a’
In the reverse reaction, energy of reactants would be more than energy of products.
Therefore, (products – reactants) would give a negative but equal value. Therefore, it would be ‘-a’