Yeah, I know, but if it was a 50 ohm isolator, then they would have had to give the length of the left 100 ohm transmission line, as the 50 ohm source impedance would immediately become a complex impedance as the line length grew from 0 degrees. Since they did not give the line length, you can not compute the impedance at the far end of the 100 ohm line. That is why I had to assume a matched generator.
Now, maybe this guy left out an important detail that the prof. wrote into the problem?
We normally think in terms of 50 ohm test equipment, but there is plenty of 75 ohm stuff out there, and even some 120 ohm stuff out there. Like I said, state your assumption right at the front and then you can argue with the prof. if he dings you.