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?