given f(x)= 4e^x -1x + 4

a) find derivative

I got 4ex -1 (which is correct)

***b) find the equation of a tangent line:

I thought that to find the equation of the tangent line you go

[f(x) - f(x+h)] / h

so i did this, plugged in

[(4ex -1 ) - (4e(x+h) -1 )] / h

cancelled and retrieved -4e for my answer.

which is wrong..

help plzz :(