Tìm n thuộc Z

$1. \mathrm{Tìm} \mathrm{n}\in \mathrm{Z}, \mathrm{biết}$
a)$\frac{-n}{4}=\frac{-9}{n}$
b)$\frac{n}{4}=\frac{18}{n+1}$​
$2. Tìm x,y\in Z,biết:$
$$\begin{gathered} a)\frac{x}{7}=\frac{9}{y} \\ b) \frac{-2}{x}=\frac{y}{5} \\ c)\frac{x-4}{y-3}=\frac{4}{3}; x - y = 5 \end{gathered}$$