[srearick]

I am having a hard time understanding this.  Any help would be wonderful.

Using Hess's Law, show how the three reactions below can be combined to obtain the standard molar enthalpy of formation of MgO(s), Mg(s) + 1/2O2(g) --> MgO(s)

Mg(s) + 2HCl(aq) --> MgCl2(aq) + H2(g)
MgO(s) + 2HCl(aq) --> MgCl2(aq) + H2O
H2(g) + 1/2O2 --> H2O(l)

[Padfoot]

Think about what your going to need on reactants side and the products side of the 3 eqns.

In Future please show an attempt.