Is this the complete problem? I mean f(6) can be anything really; I am also assuming you are meant to notice that f(x) is continuous, and thus a local maximum must occur between f(5) and f(7) - or hell f(5) and f(7) could be the local maximum and the graph dips btween f(5) and f(7). I dunno, theres a lot that can be going on here. Is this all the information - was there a picture to go along with this question?