How do I calculate t from the following equation? z=a+b(e^k*t -1) Guess this should be right to start with: t=ln(z/b) but what to do with the a and -1? and how do I calculate ln(z/b)? as lnz/lnb? I've tried thinking (didn't work *smirk*) and trial and error with simple numers (didn't work either) thanks a lot!