A parallel plate capacitor with air between the plates has a capacitance of 9 pF. The separation between its plates is '$$d$$'. The space between the plates is now filled with two dielectrics. 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