Take b = 2.

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

19 is prime.

b^((n-1)/19)-1 mod n = 1719, which is a unit, inverse 3831.

7 is prime.

b^((n-1)/7)-1 mod n = 1344, which is a unit, inverse 3436.

(7 * 19) divides n-1.

(7 * 19)^2 > n.

n is prime by Pocklington's theorem.