Hi Keith. The inverse of a ln function is an exponentiation (e) function. Here's my example a lot like your question:

f(t) = 5 - 3 ln(5t + 1)

So I "solve" for t. First I isolate the ln( ) function:
ln(5t + 1) = [5 - f] / 3

Now to eliminate the ln( ) part, I raise both sides as exponents of e:
e^{[ ln(5t + 1) ]} = e^{[ (5 - f) / 3]}

This makes the e and the ln cancel each other on the left side, dropping the 5t + 1 down:
5t + 1 = e^{[ (5 - f) / 3]}

and I continue to isolate t in the usual way:
t = [ e^{[ (5 - f) / 3]} - 1 ] / 5

Not a very pretty answer, but that is the inverse of f(t).

Now you try yours.
Cheers,
Stephen La Rocque.