a
\(\begin{gathered} \quad \quad \quad \quad \quad \quad \quad \quad A\,\,\, + \,\,\,\,\,2B\,\,\,\,\, \rightleftharpoons \,\,\,\,2C\,\,\, + \,\,\,D \hfill \\ Initially\,\,conc.\,\,\,\,\,\,\,\,a\,\,\,\,\,\,\,\,1.5a\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \hfill \\ at\,\,equllibrium\,\,\,\,a - x\,\,1.5a - 2x\,\,\,\,\,2x\,\,\,\,\,\,\,\,\,\,x \hfill \\ \end{gathered} \)
\(0.5a=x\)
\(a=2x\)
\({k_c} = \frac{{{{(2x)}^2}x}}{{(a - x){{(1.5a - 2x)}^2}}} = \frac{{4{x^2}.x}}{{(x){{(x)}^2}}} = 4\)