[tex]\displaystyle\frac{n^2-10n+24}{n^2-13n+42}-\frac{9}{n-7}=\frac{(n-6)(n-4)}{(n-6)(n-7)}-\frac{9}{n-7}\\\\=\frac{n-4}{n-7}-\frac{9}{n-7}=\frac{n-4-9}{n-7}\\\\=\frac{n-13}{n-7}[/tex]
The last choice is appropriate.
