Finding the enthalpy of formation of magnesium oxide using Hess' Law
You are trying to find out the enthalpy of formation.
The enthalpy of formation is the energy change when one mole of a substance is formed from its constituent elements in their standard state.

The last equation matches this, so that should be your 'main' or 'target' equation. From there you should be able to use Hess' law to calculate the enthalpy change of the formation of magnesium oxide.