Calculate the wavelength of the photon emitted when an electron makes a transition from n=4 to n=3. You can make use of the following constants: h=6.626×10−34 J⋅s c=2.998×108 m/s 1 m=10^9 nm
Don't you have to know either the energy of the photon or its frequency to calculate its wavelength??
Assuming you mean hydrogen (Z = 1) then:
1/lambda = Rh(1/32 - 1/42) where Rh = 1.09677576*107 m-1 is the Rydberg constant (http://hyperphysics.phy-astr.gsu.edu/hbase/hyde.html)
lambda is the wavelength
I'll leave you to plug in the numbers and rearrange to find lambda.
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