Hidden line removal

Ok. I have this problem:

Given n lines L[1...n]

3 lines cannot intersect in this problem (not sure what the reason for this is)

L: y=ax+b

L[i] is visible if there exists an X where the y-coordinant is greater than every other line's y-coordinant given the same X value.

i.e: y=2x+3 would be visible and y=2x+1 would not.

I'm supposed to develop an algorithm to determine which lines are visible (can see if looking down from y=oo) in O(nlogn)

any suggestions on how to get started???

I've came up with one algorithm in O((n^2(max-min))
where min and max are the min/max x values.. I have no idea how to do this in nlogn

I just realized that the line with the largest positive slope (a value)
and the line with the smallest negetive slope would be visible everytime.. how can i use this to do this in nlogn?