Okay. As for the definition, it informally says: 'f(x) is (or belongs to) O(g(x)) if, for all numbers bigger than some arbitrary number A the growth of g(x) is faster than or as fast as f(x)'s.'
Since the formal definition includes a constant, most authors would simply ask you to prove that 2x + 17 is O(x).
Nevertheless, to conclude your proof all you must do is show some real number such that
EDIT: Jhevon correctly pointed out that you need to specify where x is going. As the most common application of Big O is to show assymptotical domation of one funcion over another, I assumed
Hope this helps,