On transmitting a beam of X-rays with wavelength = 17.8 pm through a polycrystalline specimen a system

Question:

On transmitting a beam of X-rays with wavelength λ = 17.8 pm through a polycrystalline specimen a system of diffraction rings is produced on a screen located at a distance l = 15 cm from the specimen. Determine the radius of the bright ring corresponding to second order of reflection from the system of planes with interplanar distance d =155 pm.