How is the hess's law calculated?

Hint: Hess' law allows the enthalpy change (ΔH) for a reaction to be calculated even when it can't be measured directly. This is often accomplished by performing basic algebraic operations supporting the chemical equations of reactions using previously determined values for the enthalpies of formation.

Complete answer:
Hess's Law is known as after Russian Chemist and Doctor Germain Hess. Hess helped formulate the first principles of thermochemistry. Hess's law is thanks to enthalpy being a state function, which allows us to calculate the general change in enthalpy by simply summing the changes for every step of the way, until the product is made.
All steps need to proceed at an equivalent temperature and therefore the equations for the individual steps must balance out. The principle underlying Hess's law doesn't just apply to Enthalpy and may be wont to calculate other state functions like changes in Gibbs' Energy and Entropy.
After practicing medicine for several years in Irkutsk, Russia, Hess became professor of chemistry in 1830 at the Technological Institute, University of St. Petersburg. His early investigations concerned minerals and therefore the gas found near Baku. He discovered the oxidation of sugars to yield saccharin acid and in 1834 published a piece on chemistry that became a typical text in Russia for several years.

Note:
Hess’s law may be a consequence of the primary law of thermodynamics and wish not be considered a separate thermodynamic law, in thermochemistry, however, it retains its identity due to its importance as the basis for calculating heats of reactions.