Hess's law doesn't have specific equations, it merely states that the difference in energy between two states is immaterial of the route taken to arrive at the states.
i.e going from A to B will involve the same energy change as going from A to C to D to B
It is a statement of the law of conservation of energy.
Its utility is that if a particular energy change cannot be directly found by experimentation it can be deduced using another route.
This is done by adding and subtracting, multiplying and dividing equations (doing the same to the energy) until the required equation is produced.
see:
http://www.ibchem.com/IB/ibc/energy/energy(hl).htm
While I'm here does anyone know the official definition of Coles Law?
(mini punto for the correct answer)