6. 3 - 2/3.

$$ 3 - \frac{2}{3} = \frac{3}{1} - \frac{2}{3} = \frac{3*3}{1*3} - \frac{2}{3} = \frac{9}{3} - \frac{2}{3} = \frac{7}{3} = 2 \frac{1}{3} $$

Ответ: $$2 \frac{1}{3} $$