Which of the following expressions is modeled on the number line?

The first arrow goes from 0 to 3, which is 3 units.
The second arrow goes from 3 to 6, which is another 3 units.
The total units is 3 + 3 = 6
This means you have 2 different sets of 3, which can be modeled by 2 x 3.
The second choice is correct.