[tex] \frac{7a}{49a^2-63ab} - \frac{9a}{49a^2-81b^2} [/tex]

[tex] \frac{7a}{49a^2-63ab} - \frac{9a}{49a^2-81b^2}= \frac{a}{a(7a-9b)} - \frac{9a}{(7a-9b)(7a+9b)}= \\ =\frac{a(7a+9b)-9a*a}{a(7a-9b)(7a+9b)}=\frac{7a^2+9ab-9a^2}{a(7a-9b)(7a+9b)}=\frac{9ab-2a^2}{a(7a-9b)(7a+9b)}= \\ =\frac{9b-2a}{(7a-9b)(7a+9b)}[/tex]

[tex] \frac{9a-2b}{(7a-2b)(7a+2b)} [/tex]