Was revising the questions I got from my prep classes and

Two ways to doing this...the easy way and the right way. Since you dont get style points the easiest way on this particular on on the for you may be to write out each option and count them out. Its easy when you only use 4 jacks, if it was a picking out two face cards it would be time consuming this way. If you can't think of the right math do whatever works.

The right way is to say there are 4 jacks, since you need to make a pair and one can't pair with itself you start with N-1. Then continue on N-2 until you reach N-N. Sum up the results (N-1)+(N-2)+(N-3)..+..(N-N) = X

So N=4
4-1 = 3
4-2 = 2
4-3 = 1
4-4 = 0

So for this there are 6 options. If it was 5 then there are 10. 6 would be 15...and so on.

Hope my crazy thinking doesnt confuse you.