Having real pains too with this one.
Please see attachement - cant use the equation editior on here
I'm going to isolate Z. I assume that l = L and that ln() = Ln(). You have:
First multiply by . Afterwards expand the bracket at the RHS of the equation. You'll get:
Subtract the first summand of the RHS of the equation:
And now de-logarithmize(?):
And now try to simplify this nightmare of a solution.