Thickness of glass plate $\left(t\right)=6cm$ ; 

            Distance of the object $\left(u\right)=8cm$ ;

            And distance of the image $\left(v\right)=12cm$

            Let x = Apparent position of the silvered surface in cm

Since the image is formed due to reflection at the silvered face and by the property of mirror image.

Distance of object from the mirror = Distance of image from the mirror

Or, $x+8=12+6-x\Rightarrow x=5cm$

            Therefore, refractive index of glass = $\frac{\mathrm{Re}aldepth}{Apparentdepth}=\frac{6}{5}=1.2$