M+N=AČ, M-N=BČ

2N=AČ-BČ=(A-B)(A+B)

A-B cant equal 1, since then A+B=2N, but A-B and A+B must have the same parity. Thus A-B=2, A+B=N, -> N=2B+2, which shows that N is divisible by 2. Since N is prime, N must equal 2, which gives B=0 and then M=2.

Answer: the only pair is (2,2)