Also, we need to have the equation balanced, so be sure to remember to check for that. What we are going to do is sum up all the product enthalpies of formation and then subtract the summed up reactant enthalpies of formation.
There is a second way to use Hess' Law. So this is not a proper formation reaction. Fractional coefficients are OK.
But what are the benchmark reactions? I'll explain the above equation using an example problem. The key to solving this problem is to have a table of standard enthalpies of formation handy.