It's not in the rules, Marce, because it's an inference - something not explicitly stated that must nonetheless be true because of the actions of the rules,
In this case, if not J or K, what could be first? Never F, because it is always 2nd. Never L or M because they are always after H. Now, with G immediately
before H, neither G nor H can be in the first position - H because it is after G, and G because F gets in the way (if G was first, H would have to be second, immediately after G, but it can't be because the rules still have F second). J and K are the only variables left that could possibly be 1st, so one of them must be!
Work on making those inferences - they are the key to success in LG. They can be as simple as a not-law (like L cannot ever be first in this game - not in the rules, but because of the rules) and they can be as complicated as the G4
H1 inference we spoke about in another post.
Adam M. Tyson
PowerScore LSAT, GRE, ACT and SAT Instructor
Follow me on Twitter at https://twitter.com/LSATadam