For part (a), it looks as though you simply add to the plaintext, and then mod out . Example:
Here I've used the subscript to indicate that I'm writing the number base .
Does that make sense?
For part (b), you have to find a way to invert the operation in part (a). So, taking my example, we had better find that
using some integer . How do you undo addition? How do you undo addition modulo an integer?