How can an unknown delta h reaction be determined using hess's law?

Answer : According to the statement given in Hess's Law of Constant Heat Summation (or simply Hess's Law) tells us "that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes". 

So if any unknown Δ value needs to be found simply use the known values of Δ in the equation given below ,rearrange it accordingly to find the unknown answer and lastly add all the values to get the answer.

In simple words its the addition of multiple step enthalpy changes observed during different stages of reactions.

Example, if a reactant is directly forming a product it gives out heat in the form of Δ called as enthalpy of reaction.

Also there may be multiple other steps to form products from reactants and their heat of reactions are  Δ, Δ and Δ.

Then, Δ = Δ + Δ + Δ.