5x − 15y = 1
5(x − 3y) = 1
$x-3y=\frac{1}{5}$

$\frac{8}{2x-6y}=\frac{8}{2(x-3y)}$
т.к. $x-3y=\frac{1}{5}$, то:
$\frac{8}{2\ast \frac{1}{5}}=\frac{8}{\frac{2}{5}}=8\ast \frac{5}{2}=4\ast 5=20$

$\frac{18y-6x}{9}=\frac{6(3y-x)}{9}=-\frac{6(x-3y)}{9}$
т.к. $x-3y=\frac{1}{5}$, то:
$-\frac{6\ast \frac{1}{5}}{9}=-\frac{\frac{6}{5}}{9}=-(\frac{6}{5}\ast \frac{1}{9})=-(\frac{2}{5}\ast \frac{1}{3})=-\frac{2}{15}$

$\frac{1}{x^2-6xy+9y^2}=\frac{1}{(x-3y{)}^2}$
т.к. $x-3y=\frac{1}{5}$, то:
$\frac{1}{(\frac{1}{5}{)}^2}=\frac{1}{\frac{1}{25}}=25$