Hi LNY, I'm away from my copy of the textbook at the moment, so I can't say for sure, but doesn't the question indicate an angle of $135^\circ$ between the mirrors? It's not something I calculated, unless maybe the question gives the acute angle of $45^\circ$ between one of the mirrors and the horizontal, in which case I would have calculated it's supplement of $180^\circ - 45^\circ = 135^\circ$. Please let me know if this helps, or whether I should return to your question in a few days when I have access to the textbook.