#### Answer

$|a-b|=b-a$

#### Work Step by Step

$|a-b|,$ where $a\lt b$
The problem specifies that $a\lt b$. This means that the substraction inside the absolute value bars will result in a negative number. To express this quantity without using absolute value, we must change its sign:
$|a-b|=-(a-b)=b-a$