Question

$frac{7a}{49a^2-63ab} - frac{9a}{49a^2-81b^2}$

Answer 1

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

Answer 2

$frac{9a-2b}{(7a-2b)(7a+2b)}$