In the binomial expansion of $(a-b)^n,\:n\geq 5$ the sum of $5^{th}$ and $6^{th}$ term is $0$, then $\large\frac{a}{b}$=?

Question

$\begin{array}{1 1} \frac{6}{n-5} \\ \frac{n-5}{6} \\ \frac{5}{n-4} \\ \frac{n-4}{5} \end{array}$