A hypothetical NH molecule makes a rotational-level transition from l = 3 to l = 1 and

Question:

A hypothetical NH molecule makes a rotational-level transition from l = 3 to l = 1 and gives off a photon of wavelength 1.780nm in doing so. What is the separation between the two atoms in this molecule if we model them as point masses? The mass of hydrogen is 1.67 x 10-27 kg, and the mass of nitrogen is 2.33 x 10-26 kg.