Enthalpies of combustion( in kJ/mole) of graphite, hydrogen and ethyl alcohol are $-\mathrm{X}, -\mathrm{Y} \mathrm{and} -\mathrm{Z}$respectively.  Enthalpy of formation ( in kJ/mole) of ethyl alcohol would be

$$\begin{gathered} \mathrm{Given}.... \\ \mathrm{C}+{\mathrm{O}}_2\to {\mathrm{CO}}_2; ∆\mathrm{H}=-\mathrm{X}...\left(1\right); \\ {\mathrm{H}}_2+\frac{1}{2}{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{O}; ∆\mathrm{H}=-\mathrm{Y}...\left(2\right) \\ {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}+3{\mathrm{O}}_2\to 2{\mathrm{CO}}_2+3{\mathrm{H}}_2\mathrm{O}; ∆\mathrm{H}=-\mathrm{Z}...\left(3\right); \\ \mathrm{Required} \mathrm{equation} \mathrm{is} 2\mathrm{C}+3{\mathrm{H}}_2+\frac{1}{2}{\mathrm{O}}_2\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}; ∆\mathrm{H}=? \\ ∆\mathrm{H}=\mathrm{Eq} \left(1\right)\times 2 +\mathrm{Eq}\left(2\right)\times 3 -\mathrm{Eq}\left(3\right) \\ ∆\mathrm{H}=-2\mathrm{X}-3\mathrm{Y}-\left(-\mathrm{Z}\right) \left(\mathrm{or}\right) -2\mathrm{X}-3\mathrm{Y}+\mathrm{Z} \end{gathered}$$