Take b = 2.

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

17 is prime.

b^((n-1)/17)-1 mod n = 641, which is a unit, inverse 827.

7 is prime.

b^((n-1)/7)-1 mod n = 711, which is a unit, inverse 347.

(7^2 * 17) divides n-1.

(7^2 * 17)^2 > n.

n is prime by Pocklington's theorem.