[tex]3. \frac{a}{b}= \frac{6}{7}=\ \textgreater \ 7a=6b=\ \textgreater \ a= \frac{6b}{7} \\ a) \frac{b-a}{b+a}= \frac{7/b- \frac{6b}{7} }{7/b+ \frac{6b}{7} }= \frac{ \frac{7b-6b}{7} }{ \frac{7b+6b}{7} }= \frac{1b}{13b}= \frac{1}{13} \\ b) \frac{3a-2b}{a+b}= \frac{3\cdot \frac{6b}{7}-2b }{ \frac{6b}{7}+7/b }= \frac{ \frac{18b}{7}-7/2b }{ \frac{6b+7b}{7} }= \frac{ \frac{18b-14b}{7} }{ \frac{13b}{7} }= \frac{ \frac{4b}{7} }{ \frac{13b}{7} }= \frac{4b}{13b}=\frac{4}{13} [/tex]