In the text of the home assignment 4.1, there was a misprint concerning the units of \({{n}_{{2}}}\)  (a minus sign in the exponential)!

The correct dimensions are: \({{{m}}^{{2}}\cdot{{W}}^{{-{{1}}}}}\)    (i.e. not m^-2 W^-1)

I do apologise for the misprint. 

(One more good reason to always perform dimensionality checks...)