You can tag Bunuel and ask in Practice Questions, he is the legend for Quant..
Also, post official gmatclub links available because your description of the question seems confusing...
Here's my 2 cents:
Taking your question as drawing 4 socks out of 8 (4 pairs) and wanting both to be complete pairs.
Counting method. Total ways to pick 4 socks from 8 is 8C4 = 70. A two pair outcome is decided entirely by which 2 of the 4 pairs you end up with, so favourable cases are 4C2 = 6. Probability is 6/70 = 3/35.
Sequential works too, but it needs two cases because the second sock can either match the first or not.
Case 1. Sock 2 matches sock 1, probability 1/7. Sock 3 can be anything, sock 4 must match it, probability 1/5. This gives 1/35.
Case 2. Sock 2 does not match, probability 6/7. Sock 3 must match one of the two socks already drawn, probability 2/6. Sock 4 must then match the other one, probability 1/5. This gives 2/35.
Adding the two cases, 1/35 + 2/35 = 3/35, which matches the counting answer.
Notice that counting took one line and sequential took casework. When the question only cares about which items you end up with and not the order you drew them in, counting is usually the faster route.
If you actually meant drawing just 2 socks and getting a pair, that one is 4/8C2 = 1/7.