How would you calculate the refractive index?

[Vaish]

Homework Statement

A plano-convex lens has a focal length f= 0.2 m. The radius of curvature of the convex side of the lens has been determined to be 0.07 m and the radius of curvature of the flat side is assumed to be infinite.
Calculate the refractive index of the lens.

Homework Equations

1/f = (n-1)(1/r1-1r2)

The Attempt at a Solution

I have no idea where to start from.

 

[Drakkith]

You have $f$ and you have $r1$ and $r2$, along with the equation relating the three to the index of refraction, do you not?