$${E_n''= \left (\frac {h^2} {8m} \right) \left [ \left (\frac {1} {L^2_1} \right) n^2_x +\left (\frac {1} {L^2_1} \right) n^2_y + \left (\frac {1} {L^2_1} \right) n^2_z \right] }$$