Consider the Three-Step Mechanism for the Reaction of a Through

Short Answer

Consider the three-step mechanism for the reaction of A through intermediates B and C to produce D shown below.
A → B E_a = 15 kcal/mol, ΔH° = 13 kcal/mol
B → C E_a = 10 kcal/mol, ΔH° = -6 kcal/mol
C → D E_a = 2 kcal/mol, ΔH° = -20 kcal/mol
What's the enthalpy difference between reactant A and intermediate C?