(7 1 3 6), (9 1 7 8), (6 1 0 4), (5 1 2 ?)
(7 1 3 6) -> 7+6=13(9 1 7 8) -> 9+8=17(6 1 0 4) -> 6+4=10(5 1 2 ?) -> 5+?=12 => ?=7