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Explanation:
Draw a vertical number line. Make sure 4, 0 and -9 are marked on it.
The distance from 4 to 0 is 4 units, or 4 spaces. Let A = 4.
The distance from 0 to -9 is 9 units, or 9 spaces. Let B = 9.
Add up those distances gives a total of A+B = 4+9 = 13 inches, which is the distance from 4 to -9.
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As an alternative, we can subtract and apply absolute value
Let x = 4 and y = -9
|x-y| = |4-(-9)| = |4+9| = |13| = 13
or
|y-x| = |-9-4| = |-13| = 13
When absolute value is used, the order of subtraction doesn't matter.
