An isotope of Iodine (atomic number 53) has a mass number of 123 and a half-life of 13 hours. It decays by emitting a beta particle. If the sample has an initial activity of 2000kBq what will be the activity 52 hours later.

k2s for potassium sulphide

you'd use the equation that establishes a relationship between  energy  and  wavelength, which looks like this

e=h⋅cλ, where

λ  - the wavelength of the photon; c  - the speed of light, usually given as  3⋅108m/s; h  -  planck's constant, equal to  6.626⋅10−34j s

plug the value you were given for the energy of the photon in the above equation to get the wavelength

e=h⋅cλ⇒λ=h⋅ce

λ=6.626⋅10−34js⋅3⋅108ms−1.257⋅10−24j=1.581⋅10−1m