Incremental distance

Algebra Level 2

The distance between two numbers -2 and 7 on a number line is $7 - (-2) = 9,$ as shown above.

Now, we have three numbers $a, a^2, a^3$ on a number line, as shown below. If the distance between $a$ and $a^2$ is 6, what is the distance between $a^2$ and $a^3?$