Let K be the number field

. Find the factorizations of (7), (29) and (31) in

.

I know there's a theorem by Kronecker that says (7) is reducible iff

, has a solution (or something like that) and how to find the factorization in the case it does have a solution. But I can't seem to find a reference for this.

Can anyone suggest a reference? No spoilers to this problem please, just a reference.

Thanks