I don't understand how to calculate the material buckling.

The reference diffusion area is L^2 = 15.6187 cm^2

3. (20) Calculate the k for the system in problem 3 , using v=2.42 and k=vaf. (b) If the Neutron Lifetime of the system is 364cm2 and using the diffusion area from problem 3 , calculate the material buckling of the system. Compare with the geometric buckling from problem 3 . Is the reactor critical? Why