Looks like it is X/22
J/8---> 10/8 10-8=2 P/14---> 16/14 (+6 difference between P and J alphabet order) 16-14=2 Q/15---> 17/15 (+1) 17-15=2 W/21---> 23/21 (+6) 23-21=2 X/22---> 24/22 (+1) 24-22=2