6-21 3 =

$$6 - 2\frac{1}{3} = 5\frac{3}{3} - 2\frac{1}{3} = (5-2) + (\frac{3}{3} - \frac{1}{3}) = 3 + \frac{2}{3} = 3\frac{2}{3}$$

Ответ: $$3\frac{2}{3}$$