Now, here we have an imaginary reaction. That's a two step reaction. And each step has its corresponding change in enthalpy. Now, what Hess Law basically states is that in order to find the final change in enthalpy, you simply add each corresponding enthalpy. Together we get the final result. So ten kilojoules per mole plus 20 kilojoules per mole gives you 30 kilojoules per mole.