The wavelength is 1.875 µm.
You use the Rydberg formula to calculate the wavelength, λ:
#color(blue)(bar(ul(|color(white)(a/a) 1/λ = RZ^2(1/n_2^2 -1/n_1^2)color(white)(a/a)|)))" "#
where
#R =# the Rydberg constant (#1.097 × 10^7color(white)(l) "m"^"-1"#)
#Z =# the atomic number of the atom
#n_1# and #n_2# are the initial and final energy levels
I assume that you are referring to a hydrogen atom.
Then, we can use
#Z = 1#
#n_1 = 4#
#n_2 = 3#
#1/λ = 1.097 × 10^7color(white)(l) "m"^"-1" × 1^2 (1/3^2 -1/4^2) = 1.097 × 10^7color(white)(l) "m"^"-1" (1/9-1/16)#
#= 1.097 × 10^7color(white)(l) "m"^"-1" × (16-9)/(16×9) = 1.097 × 10^7color(white)(l) "m"^"-1" × 7/144 = 5.332 × 10^5 color(white)(l)"m"^"-1"#
#λ = 1/(5.332 × 10^5 color(white)(l)"m"^"-1") = 1.875 × 10^"-6"color(white)(l) "m" = "1.875 µm"#
