A parallel plate capacitor with air between the plates has a capacitance of $9\mu \mathrm{F}$. The separation between its plates is $d$. The space between the plates is now filled with two dielectric. One of the dielectrics has dielectric constant $K_1=3$ and thickness $\frac{d}{3}$ while the other one has dielectric constant $K_2=6$ and thickness $\frac{2d}{3}.$ Capacitance of the capacitor is now