I was going to link you to the discussion of the starting point for that (11,20), but I can't find it myself right now - this is a very active comment section.
So, since I don't know quite what part you're stuck on, obviously the first bit is that the starting point to get to 11,21 is 11,20. There's an exit from there that you can't reach normally.
If you've opened the wormhole at 11,15, then you probably already know how to reach that extra exit, and forgot the exit was there when trying to figure out 11,21.
If you haven't done that yet, then your hint is that the splitters aren't just part of a rock-pushing puzzle.