His biggest root is on the right side of the number axis.

The number on the right is greater than 0.

Such as (x-2) (x-3) (x-4) > 0.

When the number is greater than 4

(X-2)

(X-3)

(X-4) are all greater than 0.

So the product is greater than 0.

Above the number axis

Suppose x is between 3 and 4.

(X-2)

(X-3) is greater than 0

But (X-4) is less than 0.

So the product is less than 0.

beneath

So we have to go through 4 o'clock.