If the sum of $5^{th}\:and\:6^{th}$ terms in the expansion of $(a-b)^n$, $n\geq5$, is 0, then $\large\frac{a}{b}$ = ?

Question

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