If $K_1$ and $K_2$ are the respective equilibrium constants for the two reactions,
${ XeF_6(g)+H_2O(g) <=> XeOF_4(g)+2HF(g)}$
${ XeO_4(g)+XeF_6(g) <=> XeOF_4(g)+ XeO_3F_2(g)}$
The equilibrium constant of the reaction,
${XeO_4(g)+2HF(g) <=> XeO_3F_2(g)+H_2O(g)}$