Electromagnetic Waves

The electric field of a plane electromagnetic wave, travelling in an unknown nonmagnetic medium is given by,

\(E_{\mathrm{y}}=20 \sin \left(3 \times 10^6 x-4.5 \times 10^{14} \mathrm{t}\right) \mathrm{V} / \mathrm{m}\)

(where $x, \mathrm{t}$ and other values have S.I. units). The dielectric constant of the medium is $\_\_\_\_$

(speed of light in free space is $3 \times 10^8 \mathrm{~m} / \mathrm{s}$ )