Решение:

a) $$rac{7y+4}{8y} - \frac{2y+3}{6y} = \frac{3(7y+4)-4(2y+3)}{24y} = \frac{21y+12-8y-12}{24y} = \frac{13y}{24y} = \frac{13}{24}$$

б) $$rac{a}{a+4} + \frac{a}{a-4} = \frac{a(a-4)+a(a+4)}{(a+4)(a-4)} = \frac{a^2-4a+a^2+4a}{a^2-16} = \frac{2a^2}{a^2-16}$$

Ответ: a) $$\frac{13}{24}$$; б) $$\frac{2a^2}{a^2-16}$$