Find each limit$,$ if it exists. $($If an answer does $n$

$($a$)\underset{x}{lim}\frac{x^{7}4}{x^7-6}=$

$($b$)\underset{x}{lim}\frac{x^{7}4}{x^8-6}=$

$($c$)\underset{x}{lim}\frac{x^{7}4}{x^6-6}=$