Not sure how to do this problem.

In a simplified model of the human eye, the aqueous and vitreous humors and the lens all have a refractive index of 1.40, and all the refraction occurs at the cornea, whose vertex is 2.60cm from the retina. What should be the radius of curvature of the cornea such that the image of an object 40.0cm from the cornea's vertex is focused on the retina?