how do you calculate Hess's law?

Hess's Law Calculation: Refer to reactions in introduction. H1 Mg(s)+HCl(aq)MHOl2(aq)+H2g H MgO(s)+HCl(aq) H3=285.9kJ/mol H2(g)+1/2O2(g)H2O(()(heatofformationofwater) Combine H1,H2&H3 using Hess's law to find HH(MgO):Mg(s)+1/2O2(g)MgO(s) Data and Results: Part B Enthalpy of Formation of MgO In part B of this experiment, we will be finding the enthalpy of formation for MgO(s) using an indirect method. According to Hess's Law, if two or more reactions can be added to give a net reaction, H for the net reaction is simply the sum of the Hs for the reactions which are added. Consider the following three reactions: (1) Mg(s)+2H+(aq)Mg2+(aq)+H2(g)H1 (2) Mg+2(aq)+H2O(l)MgO(s)+2H+(aq)H2 (3) H2(g)+1/2O2(g)H2O(l) HH3 (4) Mg(s)+1/2O2 (g) MgO(s)H4=H1+H2+H3 Notice that when the three reactions are added, they form the formation reaction for MgO. Analogously, when we add the enthalpies for the three reactions, they give the enthalpy change associated with the formation reaction