I need help in this question; its a first order nonhomogenous differential equation. I know how to do the rest of the question but I just dont know how the book got from this part to the other. thanks!!!

(T Tinfinity (b/a)) / (Ti Tinfinity (b/a)) = exp (-at)

Hence

(T Tinfinity) / (Ti Tinfinity ) = exp (-at) + (b/a) / (Ti Tinfinity) (1- exp (-at))