а) $$3+a=\frac{5}{9}$$
$$a = \frac{5}{9} - 3 = \frac{5}{9} - \frac{27}{9} = -\frac{22}{9} = -2\frac{4}{9}$$
б) $$x+2\frac{3}{7}=5\frac{1}{7}$$
$$x = 5\frac{1}{7} - 2\frac{3}{7} = 4\frac{8}{7} - 2\frac{3}{7} = 2\frac{5}{7}$$