$C ^{\prime}= C _{1}$ and $C _{2}$ in series.
i.e. $\frac{1}{ C ^{\prime}}=\frac{1}{ C _{1}}+\frac{1}{ C _{2}}$
$\frac{1}{ C ^{\prime}}=\frac{(3 d / 4)}{\epsilon_{0} KA }+\frac{ d / 4}{\epsilon_{0} A }$
$\frac{1}{ C ^{\prime}}=\frac{ d }{4 \epsilon_{0} A }\left(\frac{3+ K }{ K }\right)$
$C ^{\prime}=\frac{4 K C _{0}}{(3+ K )}$981-s542


