∆H(white) – ∆H(reactants) = -3020 – -2900 = -120
∆ is always [final - initial], or in this case [products - reactants]. This question is about Hess's law, which is easy to understand if thought of in terms of height. ∆H(formation) is the absolute height from the ground, but ∆H(rxn) is just the distance between two points.Think of white phosphorous as the top of the empire state building; it is 3020 feet tall; it's formation releases 3020 kJ of heat. Think of red phosphorous as the third-to-last story of the empire state building; it is 2900 feet tall its formation releases 3020 kJ of heat. The distance (∆H) between the two points is 120; when going from red to white, 120 kJ must be released.
i hope that helps.