Hi I'm having trouble with this step of the derivation of the chemical potential for an ideal fermi gas in the low temperature limit

I've got as far as to find

and need to invert this expression to get the chemical potential as a function of fermi energy

I've tried writing this as (which is ok as this is the same as above to first order)

but I cant seem to get it into a form which is easily invertable any chance of a hand here please?

NB the answer I'm trying to get to is