where are the values of E1 and E2 coming from? I managed to understand that that energy formula being used will be for the particle in a box. But I don't understand where 6-3 comes from when calculating these energies someone please explain.

5. Consider 1.0010224He atoms in a box of dimensions 1.0cm1.0cm1.0cm. Calculate the occupancy of the first excited level at 1.0mK,2.0K, and 4.0K. Do the same for 3He. What conclusions might you draw from the results of your calculations? What is N0N=e(1/KT)(2E1) ? What formula