The number 7 cannot be used to write related number sentences because it wouldn't fit into the equation & make it true.
9 + 5 = 14
5 + 9 = 14
14 - 5 = 9
14 - 9 =5
Nowhere in any of these equations can the #7 go in, & make these number sentences true.
Hope this helps & makes sense ! (:
