We can the mc2 value for an electron from Table and hc=1350 eV.nm by writing as
Enx,ny,nz=2h28m(n2xL2x+n2yL2y+n2zL2z)=(hc)28(mc)2(n2xL2x+n2yL2y+n2zL2z)
For nx=ny=nz=1, we obtain
E1,1=(1240 ev.nm)8(611×103eV)(1(0.800 nm)2+1(1.600 nm)2+1(0.390 nm)2)=3.21 eV.