A lithium atom has mass 1.17 x 10-26kg, and a hydrogen atom has mass 1.67 x 10-27

Question:

A lithium atom has mass 1.17 x 10-26kg, and a hydrogen atom has mass 1.67 x 10-27 kg. The equilibrium separation between the two nuclei in the LiH molecule is 0.159 nm.
(a) What is the difference in energy between the l = 3 and l = 4 rotational levels?
(b) What is the wavelength of the photon emitted in a transition from the l = 4 to the l = 3 level?