# What is the LCD between 17/(3r-21 ) and 9/(r-7)?

In the first fraction, we can see that the denominator contains two parts being multiplied by $3$:
$\frac{17}{3 \left(r - 7\right)}$
Thus, the LCD between $3 \left(r - 7\right)$ and $\left(r - 7\right)$ is $3 \left(r - 7\right)$.