\[ \left.\frac{r^{2}-9}{r^{2}-4 r-21}=\frac{r-3}{r-7} \right\rvert\, \] Part 2 of 2 \begin{tabular}{|l|l}\hline If there is more than one restriction, use the "and" button. \\ \hline There are restrictions on the variable \( r \). \\ There are no restrictions on the variable \( r \). \\ \hline\end{tabular}

When dealing with rational expressions, it's crucial to identify any values for \( r \) that would make the denominators zero, as those values would lead to undefined expressions. Here, we set the denominator \( r^{2} - 4r - 21 \) equal to zero and factor it to find its roots, which might impose restrictions on \( r \). Additionally, the right side of the equation has a denominator of \( r - 7 \), which also needs to be considered for restrictions. By solving both conditions, we find that \( r \) cannot equal 7 or the roots derived from the quadratic equation, thus ensuring that our solution remains valid without encountering division by zero.