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Ok; you know that when an electron jumps from an allowed orbit to another it absorbs/emits energy in form of a photon of energy $E=hnu$ (where $h=$ and $nu=$frequency).

Your "red" photon represents a transition between two orbits (of $n$ and $n+1$) separated by a "energy" of:

$E_(red)=h*nu_(red)$

but red means a wavelength: $lambda_(red)=656nm$

so the "red" frequency will be: $nu_(red)=c/lambda_(red)=(3xx10^8)/(656xx10^-9)=4.57xx10^14Hz$

And energy:

$E_(red)=6.63xx10^-34*4.57xx10^14=3xx10^-19J=1.87eV$
(where $1eV=1.6xx10^-19J$)

Now we need to find the two orbits (their quantum numbers) from where and toward where the electron jumped:
We use Rydberg's Formula where we know that:
$DeltaE=E_f-E_i=1.87eV$
and also:
$DeltaE=-R(1/(n+1)^2-1/n^2)$
$1.87=-13.6(1/3^2-1/2^2)$
(I used Rydberg Constant in $eV$; $R=13.6eV$)
I found:
$1.87=1.88$ that works fine I think!

Hope it helps!

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