Take b = 2.

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

13 is prime.

b^((n-1)/13)-1 mod n = 1146, which is a unit, inverse 1577.

7 is prime.

b^((n-1)/7)-1 mod n = 2118, which is a unit, inverse 1171.

(7^2 * 13) divides n-1.

(7^2 * 13)^2 > n.

n is prime by Pocklington's theorem.