Answer:
18
Explanation:
You can do this two ways:
From -9 to 0, the distance is 9 units.
From 0 to 9, the distance is 9 units.
Add the two distances together, and you get 18 units.
You can also do this with the distance formula for a number line.
The distance, d, between numbers a and b on the number line is:
d = |a - b|
Our numbers are -9 and 9. It does not matter which number you call a and which you call b. Since the expression has absolute value in it, the answer will always come out non-negative.
d = |-9 - 9| = |-18| = 18
If you do it the other way, you get:
d = |9 - (-9)| = |9 + 9| = |18| = 18