$\begin{array}{l}2\mathrm{A}\rightleftharpoons \mathrm{B},{\mathrm{K}}_{\mathrm{eq}}=\mathrm{X}\\ 2\mathrm{B}\rightleftharpoons \mathrm{C};{\mathrm{K}}_{\mathrm{eq}}=\mathrm{Y}\\ \mathrm{D}\rightleftharpoons \mathrm{C},{\text{ K}}_{\mathrm{eq}}=\mathrm{Z}\end{array}$

Equilibrium constant for the reaction $\mathrm{D}\rightleftharpoons 4\mathrm{A}$ is

 

$\begin{array}{l}[2\mathrm{A}\rightleftharpoons \mathrm{B}]\times 2,{\mathrm{K}}_{\mathrm{eq}}={\mathrm{X}}^2\\ 2\mathrm{B}\rightleftharpoons \mathrm{C}\underset{¯}{{\mathrm{K}}_{\mathrm{eq}}}=\mathrm{Y}\\ \mathrm{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}\\ 4\mathrm{A}\rightleftharpoons \mathrm{C}{\text{\hspace{0.33em}\hspace{0.33em}\hspace{0.33em}\hspace{0.05em}K}}_{\text{eq }}={\mathrm{X}}^2\mathrm{Y}\\ \mathrm{D}\rightleftharpoons \mathrm{C}{\text{\hspace{0.33em}\hspace{0.33em}\hspace{0.33em}\hspace{0.05em}K}}_{\text{eq }}=\mathrm{Z}\\ \mathrm{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}\\ 4\mathrm{A}\rightleftharpoons \mathrm{D}{\text{\hspace{0.33em}\hspace{0.33em}\hspace{0.33em}\hspace{0.05em}K}}_{\mathrm{eq}}=\frac{{\mathrm{X}}^2\mathrm{Y}}{\mathrm{Z}}\\ \mathrm{D}\rightleftharpoons 4\mathrm{A}{\text{\hspace{0.33em}\hspace{0.33em}\hspace{0.33em}\hspace{0.05em}K}}_{\mathrm{eq}}=\frac{\mathrm{Z}}{{\mathrm{X}}^2\mathrm{Y}}\\ \mathrm{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}\end{array}$