So as x gets very, very, very large, this thing is going to approximate 4x to the 5th over 6x to the 5th for very large x. Or we could say as x approaches infinity. Now what could this be simplified to? Well, you have x to the 5th divided by x to the 5th. These are going to grow together. So you can think of them as canceling out, and so you are left with 2 thirds. So what you could say is the limit of f of x as x approaches infinity, as x gets larger and larger and larger, all of these other terms aren't going to matter that much, and so it's going to approach 2 thirds.