Hess's law by adding equations

Each equation is stored as a dictionary: products have positive numbers and reactants negative ones. A function add sums two equations and drops what cancels. You will use the burning of carbon and of carbon monoxide to find the enthalpy of formation of carbon monoxide.

The idea. The target is $\text{C} + \frac{1}{2}\text{O}_2 \rightarrow \text{CO}$. Keep the burning of carbon, and reverse the burning of CO.

Your tasks.

1. Set back to the burning of CO run backwards, and dh_back to its enthalpy change.
2. Set total to the sum of burn_c and back.
3. Set dh_total to the enthalpy change of the total.