1.
$$\frac{3x}{5} + \frac{2x}{5} = \frac{3x+2x}{5} = \frac{5x}{5} = x$$
Ответ: $$x$$
2.
$$\frac{a}{b} - \frac{3a}{b} = \frac{a-3a}{b} = \frac{-2a}{b}$$
Ответ: $$\frac{-2a}{b}$$
3.
$$\frac{2m+1}{n+2} + \frac{3m-5}{n+2} = \frac{2m+1+3m-5}{n+2} = \frac{5m-4}{n+2}$$
Ответ: $$\frac{5m-4}{n+2}$$