Hess$$'$$Law states that the heat evolved or absorbed in a chemical reaction is the same whether the process takes place in one or in several steps.

Heat evolved in the combustion of one mole $$\mathrm{C(s)}$$ in excess oxygen is $$x\,\mathrm{kJ/mol}$$, and that for the combustion of $$\mathrm{H_2(g)}$$ is $$y\,\mathrm{kJ/mol}$$ and for that of $$\mathrm{C_6H_6(l)}$$ is $$z\,\mathrm{kJ/mol}$$. Therefore, the heat change involved in the formation of one mole of $$\mathrm{C_6H_6(l)}$$ from the given equation is