Answer
$3(r-7)$
Work Step by Step
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.
$LCD=3\times(r-7)=3(r-7)$
