Part B is stumping me. I have a walkthrough of the problem but I don't understand why they do what they do. Here's the walkthrough.

Maybe I'm just missing something stupid, but how do they end up with [itex]-t/\tau[/itex]= -1? If [itex]\tau[/itex] is the time constant of the inductance (L/R), why do they "assume" or how do they calculate that t=1 and [itex]\tau[/itex]=1?