Answer:
hence initial wavelength is

Step-by-step explanation:
shift in wavelength due to compton effect is given by

λ' = the wavelength after scattering
λ= initial wave length
h= planks constant
m_{e}= electron rest mass
c= speed of light
θ= scattering angle = 180°
compton wavelength is


( put cos 180°=-1)
also given λ'=2λ
putting values and solving we get

hence initial wavelength is
