Take b = 2.

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

19 is prime. b^((n-1)/19)-1 mod n = 2, which is a unit, inverse 799.

7 is prime. b^((n-1)/7)-1 mod n = 318, which is a unit, inverse 1140.

(7 * 19) divides n-1.

(7 * 19)^2 > n.

n is prime by Pocklington's theorem.