Take b = 2.

b^(n-1) mod n = 1.

13 is prime.

b^((n-1)/13)-1 mod n = 4639, which is a unit, inverse 6238.

11 is prime.

b^((n-1)/11)-1 mod n = 4899, which is a unit, inverse 6331.

(11 * 13) divides n-1.

(11 * 13)^2 > n.

n is prime by Pocklington's theorem.