The prime factors of N are 7,7,7,3. a is the final factor of N. a may or may not be prime.
The prime factors of 66 are 3, 11. and 2
For N to be a multiple of 66, it must share prime factors with 66. N has 3 in common with 66, but is missing 2 and 11. a has to be a multiple of those missing factors. So a is not prime. The smallest a is 2*11 = 22.