To be honest, this problem was gone over symbolically in my discussion session, but I didn't fully understand it. I am having difficulties using the superposition method (or even force method) to solve these problems where a load is applied on the bar and it is constrained at both ends. I don't know where to make the cuts.
I sort of get the idea that the purpose is to remove one constraint then solve for the elongation if 1 constraint was missing, then from there I am to use that elongation to say that the elongation of the reaction force of the removed constraint is the same, which should solve the reaction force there.
For example what I wrote in my solution, I can't tell how to determine which f's to add up for the elongations, meaning why was the elongation of the first one (f1 + f2 + 2f3), elongation 2 -(f2 + 2f3) etc. I don't know how these are determined.