If C ( diamond ) → C ( graphite )+ X kJ mol^-1; C ( diamond )+ O_2( g ) → CO_2( g )+ Y kJ mol^-1; C ( graphite )+ O_2( g ) → CO_2( g )+ Z kJ mol^-1…

If $$\begin{aligned} & \mathrm{C}(\text { diamond }) \rightarrow \mathrm{C}(\text { graphite })+\mathrm{X} \mathrm{~kJ} \mathrm{~mol}^{-1} \\ & \mathrm{C}(\text { diamond })+\mathrm{O}_2(\mathrm{~g}) \rightarrow \mathrm{CO}_2(\mathrm{~g})+\mathrm{Y} \mathrm{~kJ} \mathrm{~mol}^{-1} \\ & \mathrm{C}(\text { graphite })+\mathrm{O}_2(\mathrm{~g}) \rightarrow \mathrm{CO}_2(\mathrm{~g})+\mathrm{Z} \mathrm{~kJ} \mathrm{~mol}^{-1} \end{aligned} $$ at constant temperature. Then