For a certain diatomic molecule, two of the pure-rotational absorption lines are at 806.65 GHz and 921.84

Question:

For a certain diatomic molecule, two of the pure-rotational absorption lines are at 806.65 GHz and 921.84 GHz, where 1 GHz ≡ 109 Hz, and there are no pure-rotational lines between these two lines. Find the initial J value for each of these transitions and find the molecular rotational constant B.