Overall rate of heat conduction for a fin

[theone]

Homework Statement

http://postimg.org/image/4ctgrkoop/
I don't understand the final bit, for the 'steady rate of heat transfer for the entire fin', how they went from Fourier's Law to that final expression.

Homework Equations

The Attempt at a Solution

I think it involves the previous expression but I really don't know

 

[Chestermiller]

Do you at least understand the part of the expression involving dT/dx?

Chet