To find the distance between $-9.9$ and $3.6$ on a number line, you can use the formula for the distance between two points, which is:

$\text{Distance} = |a - b|$

Where $a = -9.9$ and $b = 3.6$.

Let's compute:

$\text{Distance} = |-9.9 - 3.6| = |-9.9 - 3.6| = |-13.5| = 13.5$

So, the distance between $-9.9$ and $3.6$ is 13.5 units.

Would you like a detailed explanation or have any questions?

Here are five related questions:

1. What is the general formula for calculating the distance between two numbers on a number line?
2. How does the absolute value function help in calculating distances?
3. What happens when both numbers are negative? Does the process change?
4. Can this method be applied to fractions or decimals as well?
5. How would you represent this calculation geometrically on the number line?

Tip: When finding the distance between two points, always subtract the smaller from the larger and take the absolute value to ensure a positive result.