The left face of a biconvex lens has a radius of curvature of 12.0 cm, and the

Question:

The left face of a biconvex lens has a radius of curvature of 12.0 cm, and the right face has a radius of curvature of 18.0 cm. The index of refraction of the glass is 1.44.
(a) Calculate the focal length of the lens.
(b) Calculate the focal length if the radii of curvature of the two faces are interchanged.