Answer
When $ a\geq b\geq 0$ or $a\leq b\leq 0$
.
Work Step by Step
$|a-b|=|a|-|b| $ when:
a) a is bigger than b such that $ a-b\leq 0$
Example: $ |5-3|= |5 |- |3 |$
$2=2$
b) b is greater than a such that $ |a-b|\leq 0$
Example: $ |-5-(-3)|= |-5 |- |-3 |$
$|-2 |= 5-3$
$2=2$
