A capacitor has air as dielectric medium and two conducting plates of area $12 \mathrm{~cm}^2 and they are 0.6 \mathrm{~cm} apart. When a slab of dielectric having area 12 \mathrm{~cm}^2 and 0.6 \mathrm{~cm} thickness is inserted between the plates, one of the conducting plates has to be moved by 0.2 \mathrm{~cm} to keep the capacitance same as in previous case. The dielectric constant of the slab is : (Given \epsilon_0=8.834 \times 10^{-12} \mathrm{~F} / \mathrm{m}$)