The four cards are 5, 6, 7 and 8. Take out two cards. The possible combinations are:

5 6 、5 7 、58

6 7 、6 8

7 8

One ***6 kinds, so it's not easy to lose them if you list them.

If you ask why there aren't six fives, eight fives and eight sixes, it's because the result selected in this way is the same as that of five sixes, five eights and six eights, which is a repetition.