I just can't seem to grasp modular arithmatic and I am trying to solve a few simple proofs:

* means "is congruent"

1) Prove if [m * n (mod 7)] then [m * N (mod 14)]

2) Prove if [3 | n] then [n^(2) * 1 (mod 3)]

Also does anyone know any good sites with some basic proofs like these with solutions? Any help is appreciated.