Take b = 2.

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

17 is prime.

b^((n-1)/17)-1 mod n = 9752, which is a unit, inverse 7959.

13 is prime.

b^((n-1)/13)-1 mod n = 802, which is a unit, inverse 6991.

(13 * 17) divides n-1.

(13 * 17)^2 > n.

n is prime by Pocklington's theorem.