Answer: 4/663
Explanation:
There are 4 6s and 4 7s, so the probability of getting a 6 on the first draw is 4/52 or 1/13. After the first draw there are 51 cards remaining (given your no replacement assumption), and there are 4 7s, so the probability of getting a 7 is 4/51.
The overall probability is
1/13 x 4/51 = 4 / 663