## Intermediate Algebra (12th Edition)

$3(r-7)$
We are given the denominators $3r-21$ and $r-7$. In order to find the least common denominator, we must factor each denominator. $3r-21=3\times(r-7)$ $r-7$ cannot be simplified further Next, we multiply together all distinct factors from each denominator, with each factor raised to the greatest power that occurs in any denominator. $LCD=3\times(r-7)=3(r-7)$