4) $$\frac{5x}{a-b} - \frac{2x}{b-a} = \frac{5x}{a-b} + \frac{2x}{a-b} = \frac{5x+2x}{a-b} = \frac{7x}{a-b}$$
Ответ: $$\frac{7x}{a-b}$$