I'm not sure I understand the problem. Crossing the x-axis at 2 points means the same thing as having 2 roots, doesn't it?
If you have a continuous function f(x), and it is positive for , negative for , and then positive again for , then you know that it has at least 2 roots.
If you can tell me more, I might be able to help more. If the problem is from a book or problem set, it usually helps to copy the whole problem word-for-word. It also helps to know what you're studying.
Post again in this thread and I'll answer as soon as I can.