Some reactions cannot be run cleanly in a calorimeter — you cannot burn carbon and get pure carbon monoxide, and nobody makes acetylene from graphite by asking nicely. Hess's law gets the number anyway, and it rests on one fact: enthalpy is a state function. It depends only on where you start and where you finish, never on the route between. Climb a hill by the path or the stairs; the altitude gained is identical.

So route the target reaction through steps you DO have: $\Delta H_{\text{rxn}} = \Delta H_1 + \Delta H_2 + \Delta H_3$. Here $\Delta H_{\text{rxn}}$ is the enthalpy change of the target reaction in kJ/mol, and $\Delta H_1$, $\Delta H_2$, $\Delta H_3$ are the enthalpy changes of the routed steps in the same unit — the NUMBER subscript is a step label, first, second, third, nothing more. Two manipulations are legal, and each has a consequence you must apply to the number as well as the equation. Reverse a step and its $\Delta H$ flips sign — you are paying back what it paid out. Multiply a step by k and its $\Delta H$ multiplies by k too, because enthalpy is extensive: twice the reaction, twice the energy. Then add. The check that catches almost every slip: every species not in the target must appear on both sides of your stack and cancel. If something is left over, a step is pointing the wrong way.