Physics- how can I resolve this problem need some urgent help.?

The index of refraction of a gl varies with the wavelength of the light ping through it. This has important consequences for the design of optical instruments. A type of gl known as "extra-dense flint" has an n = 1.7378 for blue light, and an n of 1.7130 for red light. Calculate the focal length of the lens above, for both red and blue light.