The reverse of a 3-digit number abc is written cba (eg the reverse of 123 is 321).

a) Is it possible to find a 3-digit number such that the sum of itself and its reverse is prime?

b) Is it possible to find a 3-digit number such that the difference between itself and its reverse is prime?

For each question,

- if it is impossible, explain why

- if it is possible, under what conditions will the result be prime?