which exam board do you do ?
Hess’s law states that the enthalpy change for a reaction will be the same regardless of the route taken as long as start and finish conditions are the same, so maybe you could try doing the route the other way and seeing if you got the same number (although it might be a - instead of a + and vice versa)
I do OCR A Level chemistry and we’ve never had a question where you had to explain how your answer proved Hess’s Law, it’s usually just the calculation or maybe a definition of Hess’s Law, it may be different for other exam boards.