Решите 18 и 19 пжжжж

Решите задачу:

$\frac{7a}{9c}- \frac{49 a^{2}+81 c^{2} }{63ac} + \frac{9c-49a}{7a}= \frac{49 a^{2}-49 a^{2}-81 c^{2}+81 c^{2} -441ac }{63ac}=- \frac{441ac}{63ac} =-7$

$\frac{3a-7b+6}{7a-3b+6} =4\\\\ \frac{3a-7b+6}{7a-3b+6}-4=0\\\\ \frac{3a-7b+6-28a+12b-24}{7a-3b+6}=0\\\\ \frac{-25a+5b-18}{7a-3b+6} =0\\\\-25a+5b-18=0\\\\25a-5b+18=0\\\\25a-5b+22= 25a-5b+18 + 4= 0+4=4$