I was trying to use simultaneous equations but got stuck. Can anyone help me with this?

64.68 = Tf(1-exp(-200.7/Tau))

84.15 = Tf(1-exp(-243/Tau))

I got to 19.47 = 84.15*(exp-200.7/Tau) - 64.68*(exp-243/Tau)

I want to get a value for Tau but i can't figure out how to simplify/remove the exponential terms.

Any help is appreciated