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Hint

Bragg's law states that nλ = 2d sin(θ), where d is the lattice distance

the momentum tranfer is given by q = 4 π sin(θ)/λ.

now, set n=1 to get the characteristic distance: d=2π/q_first-peak

if there is only one characteristic distance (Bragg reflection), then the same distance is obtained by setting n=2 and using the second peak: d=4π/q_second-peak