The manager already hired 9 people. Let [tex] x[/tex] be the number of people he still can hire. Since these people will add to the 9 he already hired, when the hiring campaign will be over he will have hired [tex] 9+x [/tex] people. We know that he can't hire more than 14 people, so the number of people hired must be less than or equal to 14:
[tex] 9+x \leq 14 [/tex]
If we subtract 9 from both sides, we have
[tex] x \leq 14-9=5 [/tex]
so, the manager can hire at most 5 other people