Ответ:

1)

[tex] = \frac{x - y}{ - 3(x - y)} = - \frac{1}{3} [/tex]

2)

[tex] = \frac{x {}^{2}(x - 4y) }{7(4y - x)} = - \frac{x {}^{2}(x - 4y) }{7(x - 4y)} = - \frac{ {x}^{2} }{7}[/tex]

3)

[tex] = \frac{ {a}^{3}(a - 6) }{(6 - a)(6 + a)} = - \frac{ \\ {a}^{ 3}(6 - a) }{(6 - a)(6 + a) } = - \frac{ {a}^{3} }{6 + a} [/tex]