1. Use the fact that there exists such that:
You can also show that this is unique. by supposing that there exists more than one unique solution, i.e. and for some and arriving at a contradiction.
2. Here: Congruence
Thanks for the link for the second one! We just had to prove that so I guess I should have been able to figure that one out but I needed a push in the right direction!
For the first one, I'm still unsure. I actually had what you have written on my paper, but I'm still unsure of how the "b" fits into the problem. I have but I need or right? I could divide by x, giving but can I let since that's a fraction?