Hi Kiran,
rn = radius of nth orbit and r3 = radius of 3rd orbit
λ = wavelength
Formula for the radius of nth orbit, n = n^2r1
r3 = 9r1 = 9x
mv9x = 3h/2π
h/mv =6πx
So, λ = 6πx for r = 3.
Feel free to ask doubts in the Comment Section.
I hope this information helps you.
Good Luck!
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