Ray Optics Questions

The graph between angle of deviation ($\delta$) and angle of incidence ($i$) for a triangular prism is represented by:

Diameter of plano-convex lens is $6\text{cm}$ and thickness at the centre is $3\text{mm}$. If speed of light in material of lens is $2\times 10^8\text{m/s}$, the focal length of the lens is: