If measured or given X and L values are 99 mm and 154 mm respectively. Then ..
(a.)
nλ = (1/500)(99/154)
where n = integer
nλ = 0.0012857 mm
mm × 10^6 = nm
nλ = 1286 nm
(b.)
nλ = (1/1000)(99/154)
where n = integer
nλ = 0.0006428 mm
mm × 10^6 = nm
nλ = 643 nm