Number of vertices and edges lies on the boundary of bounded cells in line arrangement
Consider 3 additional lines such that their arrangement consists of a single bounded (triangular) cell $C$ and $F \subset C$. Then every edge of $F$ belongs to a zone (using a terminology from your textbook) of at least one of the 3 added lines. And all 3 zones of 3 added lines contain at most $30n$ edges according to Zone Theorem. So $F$ has no more than $30n$ edges which is $O(n)$.