Ответ:

1.

[tex]\frac{-5m}{7n^2}:\frac{m}{21n^3} =\frac{-5m}{7n^2}*\frac{21n^3}{m}=\frac{-5*21n}{7}=\frac{-5*3n}{1}=-15n[/tex]

2.

[tex]\frac{a^{2}-4b^2 }{a}:(3a-6b)= \frac{a^{2}-4b^2 }{a}*\frac{1}{3a-6b}= \frac{(a+2b)(a-2b) }{a}*\frac{1}{3(a-2b)}=\frac{a+2b}{3a}[/tex]

3.

[tex]\frac{4x-3}{x^2-6x+9}:\frac{3-4x}{x^2-3x}=\frac{4x-3}{x^2-6x+9}*\frac{x^2-3x}{3-4x}=\frac{4x-3}{(x-3)^2}*\frac{x(x-3)}{-1(4x-3)}=\frac{x}{3-x}[/tex]