Finding the enthalpy of formation of magnesium oxide using Hess' Law I have the following reactions:

$$\begin{align}
\ce{Mg(s) + 2HCl(aq) &amp;-&gt; MgCl2(aq) + H2(g)}\\
\ce{MgO(s) + 2HCl(aq) &amp;-&gt; MgCl2(aq) + H2O(aq)}\\
\ce{H2(g) + 1/2O2(g) &amp;-&gt; H2O(l)}\\
\ce{Mg(s) + 1/2O2(g) &amp;-&gt; MgO(s)}
\end{align}$$

I also have $\Delta H$ in $\mathrm{kJ/mol}$ for the first three reactions.

Out of this information, I need to find $\Delta H^\circ_\mathrm{f}{\left(\ce{MgO(s)}\right)}$.

I’m pretty sure I have to use Hess’ law but I don’t know how to apply it here. I’ve tried using the second reaction as my “main” reaction and then I can use the third and fourth reaction as formation reactions for two of the compounds of the main reaction but after that I’m not sure how to proceed.
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.