\(C + {O_2} \to C{O_2} - x\,kJ...(i)\)

\({H_2} + \frac{1}{2}{O_2} \to {H_2}O - y\,kJ...(ii)\)

\(C{H_4} + 2{O_2} \to C{O_2} + 2{H_2}O + z\,kJ...(iii)\)

The required equation is

\(C + 2{H_2} \to C{H_4} + Q\)

To get the required equation, operate \((i) + 2 \times (ii) - (iii)\)

Thus, we get

\(C + 2{H_2} \to C{H_4} + [( - x) + ( - 2y) - ( - z)]\)

Thus, heat of formation of methane is \((-x-2y+z)\,kJ\)