Thanks Aida for posting Part b in December. As there is still NO solution posted for Part d yet, I spent 1 hour to write part d and post attached same 2 images, first by camera, second by scan.
Thanks professor. Here I used error function in the textbook, instead of that given in the homework, but yes they are same. In part d, I got limit u(x,t)=1 for Neumann condition.