Water (with refractive index $=\frac{4}{3}$ ) in a tank is 18 cm deep. Oil of refractive index $\frac{7}{4}$ lies on water making a convex surface of radius of curvature ' $\mathrm{R}=6 \mathrm{~cm}$ ' as shown. Consider oil to act as a thin lens. An object ' S ' is placed 24 cm above water surface. The location of its image is at ' $x$ ' cm above the bottom of the tank. Then ' $x$ ' is