I need help in this question; it’s a first order nonhomogenous differential equation. I know how to do the rest of the question but I just don’t 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))