а) $$\frac{13}{56} + y = \frac{34}{56}$$
$$y = \frac{34}{56} - \frac{13}{56}$$
$$y = \frac{34-13}{56} = \frac{21}{56} = \frac{3}{8}$$
Ответ: $$\frac{3}{8}$$
б) $$x - \frac{7}{90} = \frac{39}{90}$$
$$x = \frac{39}{90} + \frac{7}{90}$$
$$x = \frac{39+7}{90} = \frac{46}{90} = \frac{23}{45}$$
Ответ: $$\frac{23}{45}$$