VMO 2018 Problem 6

The sequence $(x_n)$ is defined as follows: $$x_0=2, x_1=1, x_{n+2}=x_{n+1}+x_n$$ for every non-negative integer $n$ . a. For each $n\geq 1$ , prove that $x_n$ is a prime number only if $n$ is a prime number or $n$ has no odd prime divisors b. Find all non-negative pairs of integers $(m,n)$ such that $x_m|x_n$ .